Formation and Thermal Stability of Ordered Self-Assembled Monolayers by the Adsorption of Amide-Containing Alkanethiols on Au(111).

We examined the surface structure, binding conditions, electrochemical behavior, and thermal stability of self-assembled monolayers (SAMs) on Au(111) formed by N-(2-mercaptoethyl)heptanamide (MEHA) containing an amide group in an inner alkyl chain using scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and cyclic voltammetry (CV) to understand the effects of an internal amide group as a function of deposition time. The STM study clearly showed that the structural transitions of MEHA SAMs on Au(111) occurred from the liquid phase to the formation of a closely packed and well-ordered ß-phase via a loosely packed α-phase as an intermediate phase, depending on the deposition time. XPS measurements showed that the relative peak intensities of chemisorbed sulfur against Au 4f for MEHA SAMs formed after deposition for 1 min, 10 min, and 1 h were calculated to be 0.0022, 0.0068, and 0.0070, respectively. Based on the STM and XPS results, it is expected that the formation of a well-ordered ß-phase is due to an increased adsorption of chemisorbed sulfur and the structural rearrangement of molecular backbones to maximize lateral interactions resulting from a longer deposition period of 1 h. CV measurements showed a significant difference in the electrochemical behavior of MEHA and decanethiol (DT) SAMs as a result of the presence of an internal amide group in the MEHA SAMs. Herein, we report the first high-resolution STM image of well-ordered MEHA SAMs on Au(111) with a (3 × 2√3) superlattice (ß-phase). We also found that amide-containing MEHA SAMs were thermally much more stable than DT SAMs due to the formation of internal hydrogen networks in MEHA SAMs. Our molecular-scale STM results provide new insight into the growth process, surface structure, and thermal stability of amide-containing alkanethiols on Au(111).

Thermally Driven Structural Order of Oligo(Ethylene Glycol)-Terminated Alkanethiol Monolayers on Au(111) Prepared by Vapor Deposition.

To probe the effects of deposition temperature on the formation and structural order of self-assembled monolayers (SAMs) on Au(111) prepared by vapor deposition of 2-(2-methoxyethoxy)ethanethiol (CH3O(CH2)2O(CH2)2SH, EG2) for 24 h, we examined the surface structure and electrochemical behavior of the resulting EG2 SAMs using scanning tunneling microscopy (STM) and cyclic voltammetry (CV). STM observations clearly revealed that EG2 SAMs vapor-deposited on Au(111) at 298 K were composed of a disordered phase on the entire Au surface, whereas those formed at 323 K showed improved structural order, showing a mixed phase of ordered and disordered phases. Moreover, at 348 K, uniform and highly ordered EG2 SAMs on Au(111) were formed with a (2 × 3√3) packing structure. CV measurements showed sharp reductive desorption (RD) peaks at -0.818, -0.861, and -0.880 V for EG2 SAM-modified Au electrodes formed at 298, 323, and 348 K, respectively. More negative potential shifts of RD peaks with increasing deposition temperature are attributed to an increase in van der Waals interactions between EG2 molecular backbones resulting from the improved structural quality of EG2 SAMs. Our results obtained herein provide new insights into the formation and thermally driven structural order of oligo(ethylene glycol)-terminated SAMs vapor-deposited on Au(111).

Li-Ion Intercalation, Rectification, and Solid Electrolyte Interphase in Molecular Tunnel Junctions.

This paper describes Li-ion intercalation into a pyrenyl-terminated self-assembled monolayer (SAM) on gold, inspired by the graphite anode in a Li-ion battery, and its effect on tunneling performance in a molecular junction incorporating the SAM. As the concentration of the Li-ion precursor ([LiPF6]) increased from 0 to 10-2 M, the rectification ratio increased to ∼102. Further experiments revealed that the intercalation-induced changes in the orientation of PYR group and in the HOMO energy level account for the enhanced rectification. Treatment with high concentrations of LiPF6 (from 10-2 to 100 M) yielded a considerable solid electrolyte interphase (SEI), mainly composed of LiF, on the surface of the SAM, resulting in the disappearance of rectification. This was attributed to renormalization of the HOMO level back to that of the intact SAM, caused by the SEI layer. Our work demonstrates the interplay among Li-ion intercalation, SEI, and tunneling in the molecular junction, benefiting the research of molecular electronics as well as SAM-based batteries.

Case Studies on Structure-Property Relations in Perovskite Light-Emitting Diodes via Interfacial Engineering with Self-Assembled Monolayers.

Metal halide perovskites promise bright and narrow-band light-emitting diodes (LEDs). To this end, reliable understanding on structure-property relations is necessary, yet singling out one effect from others is difficult because photophysical and electronic functions of perovskite LEDs are interwoven each other. To resolve this problem, we herein employ self-assembled monolayers (SAMs) for interfacial engineering nanomaterials. Four different molecules that have the same anchor (thiol), different backbone (aryl vs alkyl) and different terminal group (amine vs pyridine vs methyl) are used to form SAMs at the interface with the thin film of a green-color perovskite, CH3NH3PbBr3. SAM-engineered perovskite films are characterized with X-ray diffraction (XRD), depth-profile X-ray photoelectron spectroscopy (XPS), Kelvin probe force microscopy (KPFM), scanning electron microscopy (SEM), time-resolved laser spectroscopy, and UV-vis absorption and emission spectroscopies. This permits access to how the chemical structure of molecule comprising SAM is related to the various chemical and physical features such as quality and grain size, cross-sectional atomic composition (Pb(0) vs Pb(II)), charge carrier lifetime, and charge mobility of perovskite films, leading to inferences of structure-property relations in the perovskite. Finally, we demonstrate that the trends observed in the model system stem from the affinity of SAM over the undercoordinated Pb ions of perovskite, and these are translated into considerably enhanced EQE (from 2.20 to 5.74%) and narrow-band performances (from 21.3 to 15.9 nm), without a noticeable wavelength shift in perovskite LEDs. Our work suggests that SAM-based interfacial engineering holds a promise for deciphering mechanisms of perovskite LEDs.

Interstitially Mixed Self-Assembled Monolayers Enhance Electrical Stability of Molecular Junctions.

Electrical breakdown is a critical problem in electronics. In molecular electronics, it becomes more problematic because ultrathin molecular monolayers have delicate and defective structures and exhibit intrinsically low breakdown voltages, which limit device performances. Here, we show that interstitially mixed self-assembled monolayers (imSAMs) remarkably enhance electrical stability of molecular-scale electronic devices without deteriorating function and reliability. The SAM of the sterically bulky matrix (SC11BIPY rectifier) molecule is diluted with a skinny reinforcement (SCn) molecule via the new approach, so-called repeated surface exchange of molecules (ReSEM). Combined experiments and simulations reveal that the ReSEM yields imSAMs wherein interstices between the matrix molecules are filled with the reinforcement molecules and leads to significantly enhanced breakdown voltage inaccessible by traditional pure or mixed SAMs. Thanks to this, bias-driven disappearance and inversion of rectification is unprecedentedly observed. Our work may help to overcome the shortcoming of SAM's instability and expand the functionalities.

Improved Stability of All-Polymer Solar Cells Using Crosslinkable Donor and Acceptor Polymers Bearing Vinyl Moieties in the Side-Chains.

Crosslinkable polymers have attracted tremendous attention in various fields of science and technology, owing to their potential utilization in applications requiring dimensional and morphological stability under thermal and mechanical stress. In this study, random terpolymers were successfully synthesized by introducing thiophene-based monomers bearing vinyl functional groups in the side-chain of the polymer donor (PBDBT-BV20) and polymer acceptor (N2200-TV10) structures. The physical properties of the blend films of PBDBT-BV20 and N2200-TV10 before and after thermal crosslinking were extensively investigated and compared to those of the homogeneous individual polymer films. The results revealed that a network polymer with donor and acceptor polymer chains, which can lock the internal morphology, could be achieved by inducing crosslinking between the vinyl groups in the mixed state of PBDBT-BV20 and N2200-TV10. In addition, the power conversion efficiency (PCE) of the polymer solar cells (PSCs) containing the blend films that were crosslinked by a two-step thermal annealing process was improved. The enhanced PCE could be attributed to the individual crystallization of PBDBT-BV20 and N2200-TV10 in the blend phase at 120 °C and then thermal crosslinking at 140 °C. In addition, the PSCs with the crosslinked blend film exhibited an excellent shelf-life of over 1200 h and a thermally stable PCE. Furthermore, the crosslinked blend film exhibited excellent mechanical stability under bending stress in flexible PSCs using plastic substrates.

Nonhalogenated Solvent-Processed High-Performance Indoor Photovoltaics Made of New Conjugated Terpolymers with Optimized Monomer Compositions.

Conjugated random terpolymers, PJ-25, PJ-50, and PJ-75 were successfully synthesized from three different monomers. Fluorine-substituted benzotriazole (2F-BTA) was incorporated into 4,8-bis(4-chlorothiophen-2-yl)benzo[1,2-b:4,5-b']dithiophene (BDT-T-Cl) and a 1,3-bis(4-(2-ethylhexyl)thiophen-2-yl)-5,7-bis(2-alkyl)benzo[1,2-c:4,5-c']dithiophene-4,8-dione (BDD)-based alternating copolymer PM7 as a third monomeric unit. The solubility of the random terpolymers in nonhalogenated solvents increased with the number of 2F-BTA units in PM7. The random terpolymers were mixed with 3,9-bis(2-methylene-((3-(1,1-dicyanomethylene)-6,7-difluoro)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2',3'-d']-s-indaceno[1,2-b:5,6-b']dithiophene (IT-4F) to fabricate organic photovoltaic (OPV) cells. Among the three terpolymers and two related binary copolymers (e.g., PM7 and J52-Cl), outdoor photovoltaic (PV) cells (AM 1.5G) based on the PJ-50:IT-4F blend showed a high power conversion efficiency (PCE) of 11.34%. In addition, PJ-50 was employed as a donor in indoor PV (IPV) cells and was blended with nonfullerene acceptors, which have different absorption ranges. Among them, the PJ-50:IT-4F-based IPV device had the highest PCE of 17.41% with a Jsc of 54.75 µA cm-2 and an FF of 0.77 under 160 µW cm-2 light-emitting diode (LED) light. The terpolymer introduced in this study can be regarded as a promising material for the fabrication of outdoor PV and IPV cells with excellent performance involving the use of an eco-friendly solvent.

Achieving Ultralow, Zero, and Inverted Tunneling Attenuation Coefficients in Molecular Wires with Extended Conjugation.

Molecular tunnel junctions are organic devices miniaturized to the molecular scale. They serve as a versatile toolbox that can systematically examine charge transport behaviors at the atomic level. The electrical conductance of the molecular wire that bridges the two electrodes in a junction is significantly influenced by its chemical structure, and an intrinsically poor conductance is a major barrier for practical applications toward integrating individual molecules into electronic circuitry. Therefore, highly conjugated molecular wires are attractive as active components for the next-generation electronic devices, owing to the narrow highest occupied molecular orbital-lowest occupied molecular orbital gaps provided by their extended π-building blocks. This article aims to highlight the significance of highly conductive molecular wires in molecular electronics, the structures of which are inspired from conductive organic polymers, and presents a body of discussion on molecular wires exhibiting ultralow, zero, or inverted attenuation of tunneling probability at different lengths, along with future directions.

Solid State Dilution Controls Marcus Inverted Transport in Rectifying Molecular Junctions.

Traditional Marcus theory accounts for electron transfer reactions in solutions, and the polarity of solvent molecule matters for them. How such an environment polarity affects electron transfer reactions in solid-state devices, however, remains uncertain. This paper describes how the Marcus inverted charge transport is influenced by solid-state molecular dilution in large-area tunneling junctions. A monolayer of 2,2'-bipyridyl terminated n-alkanethiolate (SC11BIPY), which rectifies currents via electron hopping within the inverted regime, is diluted with n-alkanethiolate (SCn) of different lengths (n = 8, 10, or 18) or at different surface mole fractions. The dilution introduces nonpolar environments within the monolayer, hinders stabilization of charged BIPY species upon electron hopping, and pushes the equilibrium of BIPY ⇄ BIPY•- process toward the reverse direction. Our work demonstrates that solid-state molecular dilution permits systematic control of the environment polarity of active component in nanoscale devices, much like solvent polarity control in solution, and their performances.

Superexchange Coupling-Induced Enhancements of Thermoelectric Performance in Saturated Molecules.

To develop thermoelectric devices, it is of the utmost importance to design organic building blocks to have efficient thermopower. Whereas conjugated and aromatic molecules with intrinsic narrow band gaps are attractive candidates to achieve efficient thermoelectric properties, saturated molecules are usually avoided owing to intrinsically poor thermopower. Here we demonstrate that thermopower of saturated molecules can be enhanced by superexchange coupling. Specifically, thermoelectric properties of large-area junctions that contain self-assembled monolayers of oligo(ethylene glycol) thiolates and alkanethiolates are compared. Through large-area thermopower measurements using a liquid metal top electrode, we show that the superexchange coupling enhances the Seebeck coefficient and counterintuitively leads to an increase in the Seebeck coefficient with increasing the length in a certain conformation. The improved thermoelectric performance is attributed to the superexchange-induced enhanced ability to mediate metal wave function in junctions. Our work offers new insights for improving the thermoelectric performance of nonconjugated, saturated molecules.

Interplay of Fermi Level Pinning, Marcus Inverted Transport, and Orbital Gating in Molecular Tunneling Junctions.

This Letter examines the interplay of important tunneling mechanisms-Fermi level pinning, Marcus inverted transport, and orbital gating-in a molecular rectifier. The temperature dependence of the rectifying molecular junction containing 2,2'-bipyridyl terminated n-alkanethiolate was investigated. A bell-shaped trend of activation energy as a function of applied bias evidenced the dominant occurrence of unusual Marcus inverted transport, while retention of rectification at low temperatures implied that the rectification obeyed the resonant tunneling regime. The results allowed reconciling two separately developed transport models, Marcus-Landauer energetics and Fermi level pinning-based rectification. Our work shows that the internal orbital gating can be substituted with the pinning effect, which pushes the transport mechanism into the Marcus inverted regime.

Tunneling and thermoelectric characteristics of N-heterocyclic carbene-based large-area molecular junctions.

Reported herein are tunneling and thermoelectric characteristics of large-area molecular junctions formed with N-heterocyclic carbene (NHC)-based self-assembled monolayers on gold.

Single-Crystal Poly[4-(4,4-dihexadecyl-4H-cyclopenta[1,2-b:5,4-b']dithiophen-2-yl)- alt-[1,2,5]thiadiazolo[3,4- c]pyridine] Nanowires with Ultrahigh Mobility.

We fabricated single-crystal poly[4-(4,4-dihexadecyl-4H-cyclopenta[1,2-b:5,4-b']-dithiophen-2-yl)- alt-[1,2,5]thiadiazolo-[3,4- c]pyridine] (PCDTPT) nanowires with ultrahigh mobility using a liquid-bridge-mediated nanotransfer molding method. The structural analysis of the single-crystal PCDTPT nanowires reveals that PCDTPT crystals have a triclinic structure, and the nanowires grow parallel to PCDTPT backbone chains, which provide important insights into its intrinsic charge transport. The single-crystal PCDTPT nanowire exhibits a superior charge carrier mobility of 72.94 ± 18.02 cm2 V-1 s-1 (maximum mobility up to 92.64 cm2 V-1 s-1), which is a record high value among conjugated polymers to date. In the single-crystal PCDTPT nanowire, the backbone chains in the linear structure along the nanowire growth axis lead to strong backbone delocalization, resulting in highly conductive polymer backbones and a drastic increase in charge carrier mobility. In addition, the single-crystal PCDTPT nanowire shows good environmental stability under air conditions compared to small-molecule organic semiconductors.

Striped Phase of 3-Hexylthiophene Self-Assembled Monolayers on Au(1 11) Formed by Vapor Phase Deposition.

The formation and surface structure of 3-hexylthiophene (HTP) self-assembled monolayers (SAMs) on Au(111) prepared by solution and ambient-pressure vapor deposition at room temperature (RT) for 24 h were examined by means of scanning tunneling microscopy (STM) and cyclic voltammetry (CV). STM imaging revealed that HTP SAMs formed by solution deposition have a disordered phase, whereas those formed by vapor deposition exhibit a striped phase with a unidirectional orientation. The distance between the rows in the striped phase was measured to be 1.3 ± 0.1 nm, and the hexyl molecular backbones of HTP in the SAMs on Au(111) are oriented parallel to the Au(111) surface with the head-to-head orientation. From this STM observation, we suggest that the formation of this striped phase in HTP SAMs prepared by vapor deposition were mainly driven by the optimization of van der Waals interactions between the hexyl chains on the surface. CV measurements also demonstrated that HTP SAMs show a high blocking efficiency for electron transfer reactions between electrolytes and the gold electrode, suggesting the formation of SAMs on Au(111) from the vapor phase. Our results obtained here will be very useful for understanding the formation and structure of HTP SAMs on Au(111) surfaces and how they are influenced by deposition method.

Formation of Ordered 4-Fluorobenzenethiol Self-Assembled Monolayers on Au(111) from Vapor Phase Deposition.

Self-assembled monolayers (SAMs) were formed by the spontaneous adsorption of 4-fluorobenzenethiol (4-FBT) on Au(111) using both solution and ambient-pressure vapor deposition methods at room temperature. The surface structure and thermal desorption properties of 4-FBT SAMs were examined by scanning tunneling microscopy (STM) and thermal desorption spectroscopy (TDS). STM imaging showed that 4-FBT SAMs formed in solution at room temperature mainly contained disordered phase with gold adatom islands, while those formed by ambient-pressure vapor deposition had well-ordered phase, which can be described as a (2 x 2√13)R45 degrees structure. In addition, thermal desorption spectroscopy (TDS) measurements showed that strong desorption peak for parent mass fragment (m/z = 128, FC6H5SH+) for 4-FBT SAMs on Au(111) was observed at 460 K, as a result of hydrogen abstract reaction of chemisorbed thiolates during desorption.

Effect of Solution Concentration on the Formation of Ordered Domains in Pentachlorobenzenethiol Self-Assembled Monolayers on Au(111).

The surface structures of self-assembled monolayers (SAMs) formed by the adsorption of pentachlorobenzenethiol (PCBT) molecules on Au(111) as a function of solution concentration were examined by means of scanning tunneling microscopy (STM) to understand the effect of concentration on the formation of ordered domains. STM imaging revealed that PCBT SAMs formed in a 0.01 or 1 mM ethanol solution at room temperature for 20 min contained small ordered domains in the range of several to 20 nm2 and disordered phases, while PCBT SAMs formed in a 0.1 mM ethanol solution were composed of long-range ordered domains in the range of 20 to 50 nm2, which can be assigned as a (4 x √3)R45 degrees packing structure. Interestingly, the bright aggregated domains stacked by π-π interactions between PCBT rings were usually observed around boundary regions of ordered domains. In addition, X-ray photoelectron spectroscopy measurements revealed that ordered PCBT SAMs on Au(111) were formed via the chemical interactions between the sulfur atom of PCBT and gold surface. Our results obtained here will be very useful in understanding the formation and structure of PCBT SAMs on gold surfaces.

Formation and structure of ordered pentachlorobenzenethiol self-assembled monolayers on Au(111) studied by scanning tunneling microscopy.

Molecular-scale surface structures of self-assembled monolayers (SAMs) prepared by the adsorption of pentafluorobenzenethiols (PFBT) and pentachlorobenzenethiols (PCBT) on Au(111) were investigated by scanning tunneling microscopy (STM). High-resolution STM imaging revealed that PFBT SAMs on Au(111) have long-range ordered domains with a row structure at room temperature, whereas PCBT SAMs have small ordered domains, with disordered domains as the main phase. This may reflect the larger diffusion barriers of PCBT molecules on Au(111) surfaces compared to PFBT molecules during SAM formation. The structural transitions of PCBT SAMs from the mixed phase containing disordered and ordered domains to the uniform ordered domains were observed at 50 degrees C depending on immersion time. The ordered packing structure of PCBT SAMs is an incommensurate (square root of 3 x square root of 10)R45 degrees structure, which differs from that of PFBT SAMs with a (2 x 5 square root of 13)R30 degrees structure. We found that a small modification in the chemical structures of aromatic rings using a halo-substituent strongly affects the self-assembly mechanism and packing structure of aromatic thiol SAMs on Au(111). Moreover, we demonstrated that highly ordered PCBT SAMs can be obtained at a solution temperature of 50 degrees C after immersion for 60 min.

Effect of molecular desorption on the electronic properties of self-assembled polarizable molecular monolayers.

We investigated the interfacial electronic properties of self-assembled monolayers (SAM)-modified Au metal surface at elevated temperatures. We observed that the work functions of the Au metal surfaces modified with SAMs changed differently under elevated-temperature conditions based on the type of SAMs categorized by three different features based on chemical anchoring group, molecular backbone structure, and the direction of the dipole moment. The temperature-dependent work function of the SAM-modified Au metal could be explained in terms of the molecular binding energy and the thermal stability of the SAMs, which were investigated with thermal desorption spectroscopic measurements and were explained with molecular modeling. Our study will aid in understanding the electronic properties at the interface between SAMs and metals in organic electronic devices if an annealing treatment is applied.

Spontaneous desorption and phase transitions of self-assembled alkanethiol and alicyclic thiol monolayers chemisorbed on Au(111) in ultrahigh vacuum at room temperature.

Scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) were used to examine the surface structure and adsorption conditions of hexanethiol (HT) and cyclohexanethiol (CHT) self-assembled monolayers (SAMs) on Au(111) as a function of storage period in ultrahigh vacuum (UHV) conditions of 3×10(-7) Pa at room temperature (RT). STM imaging revealed that after storage for 7 days, HT SAMs underwent phase transitions from c(4×2) phase to low coverage 4×√3 phase. This transition is due to a structural rearrangement of hexanethiolates that results from the spontaneous desorption of chemisorbed HT molecules on Au(111) surface. XPS measurements showed approximately 28% reduction in sulfur coverage, which indicates desorption of hexanethiolates from the surfaces. Contrary to HT SAMs, the structural order of CHT SAMs with (5×2√3)R35° phase completely disappeared after storage for 3 or 7 days. XPS results show desorption of more than 80% of the cyclohexanethiolates, even after storage for 3 days. We found that spontaneous desorption of CHT molecules on Au(111) in UHV at RT occurred quickly, whereas spontaneous desorption of HT molecules was much slower. Thermal desorption spectroscopy (TDS) results suggest CHT SAMs in UHV at RT can desorb more efficiently than HT SAMs due to formation of thiol desorption fragments that result from chemical reactions between surface hydrogen atoms and thiolates on Au(111) surfaces. This study clearly demonstrated that organic thiols chemisorbed on gold surfaces are desorbed spontaneously in UHV at RT and van der Waals interactions play an important role in determining the structural stability of thiolate SAMs in UHV.

An alkyl spacer effect on the structure of 4-fluorobenzenethiol and 4-fluorobenzenemethanethiol self-assembled monolayers on Au(111).

Self-assembled monolayers (SAMs) formed by the adsorption of 4-fluorobenzenethiol (4-FBT) and 4-fluorobenzenemethanethiol (4-FBMT) on Au(111) were examined by scanning tunneling microscopy (STM) to understand the effect of a flexible methylene spacer between the sulfur head-group and phenyl group and the effect of solution temperature on the formation and structure of the SAMs. Although the adsorption of 4-FBT on Au(111) at room temperature for 24 h generated only disordered phase SAMs containing gold adatom islands, 4-FBT at 75 degrees C for 2 h formed mixed SAMs: disordered phases and ordered (2 x 12√(2))R10 degrees superlattice with a rectangular unit cell containing six adsorbed molecules. On the other hand, SAMs formed from 4-FBMT, with a methylene spacer, at room temperature for 24 h on Au(111) had irregularly ordered phases containing uniformly distributed VIs with lateral dimensions of 2-5 nm; SAMs formed from 4-FBMT at 75 degrees C for 2 h were composed of slightly improved ordered phases and larger VIs with lateral dimensions of 5-12 nm as a result of Ostwald ripening. From this study, we found that the methylene spacer plays an important role in controlling the structure of SAMs formed from p-substituted fluorinated aromatic thiols.