Depolarization Effects in Self‐Assembled Monolayers: A Quantum‐Chemical Insight

Many recent experimental studies have demonstrated that the deposition of a self‐assembled monolayer (SAM) made of polar molecules on a metal surface can significantly modulate its work function and hence the barrier for hole and electron injection in optoelectronic devices. The permanent dipole moment associated with the backbone of the molecules plays a key role in defining the amplitude and direction of the work‐function shift. We illustrate here via quantum‐chemical calculations performed on model systems that the dipole moment of molecules is significantly reduced going from the isolated state to the SAM. Such depolarization effects that are most often neglected thus reduce the work‐function shift and have to be taken in account to control and understand charge‐injection barriers in devices at a quantitative level.     

Formation of Metal Nano‐ and Micropatterns on Self‐Assembled Monolayers by Pulsed Laser Deposition Through Nanostencils and Electroless Deposition

Patterns of noble‐metal structures on top of self‐assembled monolayers (SAMs) on Au and SiO₂ substrates have been prepared following two approaches. The first approach consists of pulsed laser deposition (PLD) of Pt, Pd, Au, or Cu through nano‐ and microstencils. In the second approach, noble‐metal cluster patterns deposited through nano‐ and microstencils are used as catalysts for selective electroless deposition (ELD) of Cu. Cu structures are grown on SAMs on both Au and SiO₂ substrates and are subsequently analyzed using X‐ray photoelectron spectroscopy element mapping, atomic force microscopy, and optical microscopy. The combination of PLD through stencils on SAMs followed by ELD is a new method for the creation of (sub)‐micrometer‐sized metal structures on top of SAMs. This method minimizes the gas‐phase deposition step, which is often responsible for damage to, or electrical shorts through, the SAM.     

Dip‐Pen Nanolithography Using the Amide‐Coupling Reaction with Interchain Carboxylic Anhydride‐ Terminated Self‐Assembled Monolayers

Herein we report on a new type of dip‐pen nanolithography (DPN), which utilizes an interfacial organic reaction—the amide‐coupling reaction—between chemically activated surfaces and amine ink molecules transferred from an atomic force microscopy tip. As a representative of the chemically activated surfaces that could react with amine compounds, we formed a self‐assembled monolayer terminating in interchain carboxylic anhydride (ICA) groups on gold, and generated chemically derived nanopatterns using alkylamines as ink molecules. Amine inks showed diffusive behavior similar to thiol inks on gold in conventional DPN, and the pattern sizes were controlled by changing the tip dwell times. In addition, nanopatterns of hydrolyzed ICAs were generated by taking advantage of the participation of the water meniscus in the DPN process and the chemical nature of the ICAs.     

Surface‐Transfer Doping of Organic Semiconductors Using Functionalized Self‐Assembled Monolayers

Controlling charge doping in organic semiconductors represents one of the key challenges in organic electronics that needs to be solved in order to optimize charge transport in organic devices. Charge transfer or charge separation at the molecule/substrate interface can be used to dope the semiconductor (substrate) surface or the active molecular layers close to the interface, and this process is referred to as surface‐transfer doping. By modifying the Au(111) substrate with self‐assembled monolayers (SAMs) of aromatic thiols with strong electron‐withdrawing trifluoromethyl (CF₃) functional groups, significant electron transfer from the active organic layers (copper(II) phthalocyanine; CuPc) to the underlying CF₃‐SAM near the interface is clearly observed by synchrotron photoemission spectroscopy. The electron transfer at the CuPc/CF₃‐SAM interface leads to an electron accumulation layer in CF₃‐SAM and a depletion layer in CuPc, thereby achieving p‐type doping of the CuPc layers close to the interface. In contrast, methyl (CH₃)‐terminated SAMs do not display significant electron transfer behavior at the CuPc/CH₃‐SAM interface, suggesting that these effects can be generalized to other organic‐SAM interfaces. Angular‐dependent near‐edge X‐ray absorption fine structure (NEXAFS) measurements reveal that CuPc molecules adopt a standing‐up configuration on both SAMs, suggesting that interface charge transfer has a negligible effect on the molecular orientation of CuPc on various SAMs.     

Cover Picture: Formation of Metal Nano‐ and Micropatterns on Self‐Assembled Monolayers by Pulsed Laser Deposition Through Nanostencils and Electroless Deposition (Adv. Funct. Mater. 10/2006)

The cover illustrates two‐step fabrication of metal micro‐ and nanostructures on self‐assembled monolayers (SAMs) by pulsed laser deposition and electroless deposition. Metal–SAM–metal junctions are a key component of molecular electronic devices. Pt was deposited in a micropattern by pulsed laser deposition through a stencil. XPS maps show how the Pt pattern is developed into a Cu pattern using electroless deposition as reported by Ravoo, Brugger, Reinhoudt, Blank, and co‐workers on p. 1337. The Cu pattern can also be observed by optical microscopy (background). Patterns of noble‐metal structures on top of self‐assembled monolayers (SAMs) on Au and SiO₂ substrates have been prepared following two approaches. The first approach consists of pulsed laser deposition (PLD) of Pt, Pd, Au, or Cu through nano‐ and microstencils. In the second approach, noble‐metal cluster patterns deposited through nano‐ and microstencils are used as catalysts for selective electroless deposition (ELD) of Cu. Cu structures are grown on SAMs on both Au and SiO₂ substrates and are subsequently analyzed using X‐ray photoelectron spectroscopy element mapping, atomic force microscopy, and optical microscopy. The combination of PLD through stencils on SAMs followed by ELD is a new method for the creation of (sub)‐micrometer‐sized metal structures on top of SAMs. This method minimizes the gas‐phase deposition step, which is often responsible for damage to, or electrical shorts through, the SAM.     

Low‐Operating‐Voltage Organic Transistors Made of Bifunctional Self‐Assembled Monolayers

Self‐assembled monolayers (SAMs) are molecular assemblies that spontaneously form on an appropriate substrate dipped into a solution of an active surfactant in an organic solvent. Organic field‐effect transistors are described, built on an SAM made of bifunctional molecules comprising a short alkyl chain linked to an oligothiophene moiety that acts as the active semiconductor. The SAM is deposited on a thin oxide layer (alumina or silica) that serves as a gate insulator. Platinum–titanium source and drain electrodes (either top‐ or bottom‐contact configuration) are patterned by using electron‐beam (e‐beam) lithography, with a channel length ranging between 20 and 1000 nm. In most cases, ill‐defined current–voltage (I–V) curves are recorded, attributed to a poor electrical contact between platinum and the oligothiophene moiety. However, a few devices offer well‐defined curves with a clear saturation, thus allowing an estimation of the mobility: 0.0035 cm² V^–1 s^–1 for quaterthiophene and 8 × 10^–4 cm² V^–1 s^–1 for terthiophene. In the first case, the on–off ratio reaches 1800 at a gate voltage of –2 V. Interestingly, the device operates at room temperature and very low bias, which may open the way to applications where low consumption is required.     

Patterned Self‐Assembled Monolayers on Silicon Oxide Prepared by Nanoimprint Lithography and Their Applications in Nanofabrication

Nanoimprint lithography (NIL) is used as a tool to pattern self‐assembled monolayers (SAMs) on silicon substrates because of its ability to pattern in the micrometer and nanometer ranges. The polymer template behaves as a physical barrier preventing the formation of a SAM in the covered areas of the substrate. After polymer removal, SAM patterns are obtained. The versatility of the method is shown in various nanofabrication schemes. Substrates are functionalized with a second type of silane adsorbate. Pattern enhancement via selective electrostatic attachment of carboxylate‐functionalized particles is achieved. Further applications of the NIL‐patterned substrates include template‐directed adsorption of particles, as well as the fabrication of electrodes on top of a SAM.     

Intramolecular Dipole Coupling and Depolarization in Self‐Assembled Monolayers

Quantum mechanical and classical atomistic computational methods are used to simulate the chain‐length dependence of depolarization effects in S(CH₂)_n?1CH₃ and S(CH₂)_n?1COOH self‐assembled monolayers on gold (111) surface. These calculations show that due to weak cooperative effects, the electrostatic properties of alkanethiol monolayers are well described by the gas phase dipole moments of the molecules. However, depolarization in monolayers with the molecules carrying head‐ and tail‐group dipoles, such as COOH‐terminated monolayers, strongly depends on the degree of intramolecular dipole coupling. Thus the electrostatic properties of self‐assembled monolayers can be engineered by changing the length of the aliphatic spacer between the polar groups. The transition from strong to weak coupling regime was found to be accompanied by the change in the sign of the asymptotic value of electrostatic potential above the surface of the monolayers and hence in the sign of the metal work function change. Therefore, the use of weakly polarizable spacers between the polar groups inside the molecules forming the SAM is beneficial for accessing a wider range of work‐function changes.     

The Effects of Embedded Dipoles in Aromatic Self‐Assembled Monolayers

Using a representative model system, here electronic and structural properties of aromatic self‐assembled monolayers (SAMs) are described that contain an embedded, dipolar group. As polar unit, pyrimidine is used, with its orientation in the molecular backbone and, consequently, the direction of the embedded dipole moment being varied. The electronic and structural properties of these embedded‐dipole SAMs are thoroughly analyzed using a number of complementary characterization techniques combined with quantum‐mechanical modeling. It is shown that such mid‐chain‐substituted monolayers are highly interesting from both fundamental and application viewpoints, as the dipolar groups are found to induce a potential discontinuity inside the monolayer, electrostatically shifting the core‐level energies in the regions above and below the dipoles relative to one another. These SAMs also allow for tuning the substrate work function in a controlled manner independent of the docking chemistry and, most importantly, without modifying the SAM‐ambient interface.     

Electropolymerized Self‐Assembled Monolayers of a 3,4‐Ethylenedioxythiophene‐Thiophene Hybrid System

Self‐assembled monolayers (SAMs) of a conjugated bithiophenic system connected to an alkanethiol chain have been deposited on gold surface. The electroactive bithiophenic system involves a 3,4‐ethylenedioxythiophene (EDOT) unit and a thiophene ring on which an alkanethiol is attached at the internal β‐position via a sulfide linkage. The analysis of the structure of the SAMs by IR spectroscopy, ellipsometry, contact angle measurement and X‐ray photoelectron spectroscopy (XPS) provides consistent results indicating compact monolayers in which the alkyl linkers are arranged in an almost vertical fashion while the bithiophenic‐conjugated systems are essentially parallel to the surface. Cyclic voltammetry shows that application of a few potential scans to SAMs immersed in a medium containing only a supporting electrolyte leads to the typical electropolymerization curves while the CV of the electrooxized monolayer exhibits a reversible cyclic voltammogram characteristic of a stable electroactive extended conjugated system. The characterization of the electropolymerized monolayers by IR spectroscopy, ellipsometry, contact angle measurement, and XPS indicates compact monolayers. The analysis of the current voltage characteristics of the monolayers by conducting AFM before and after electrooxidation shows that the enhancement of the effective conjugation resulting from electropolymerization leads to a significant increase of the transport properties.     

Self‐Assembled Monolayer Coatings on Nanostencils for the Reduction of Materials Adhesion

Nanostencils (shadow masks with submicrometer apertures in a thin silicon nitride membrane) are promising tools for the facile one‐step generation of nanopatterns of various materials by physical vapor deposition. Evaporation through a shadow mask is accompanied by gradual clogging of the apertures due to adhesion of evaporated material. In order to reduce this effect, nanostencils were coated with alkyl and perfluoroalkyl self‐assembled monolayers (SAMs). The formation and properties of SAMs on planar silicon nitride substrates were studied by contact angle goniometry, X‐ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM). The SAMs are stable under evaporation of gold at various angles. SAM‐coated nanostencils showed considerably less adhesion of gold compared to bare Si_xN_y stencils.     

Arsonic Acid Self‐Assembled Monolayers Protect Oxide Surfaces from Micronewton Nanomechanical Forces

The development of new surface coatings is critical for combating wear and increasing the device lifetime in microelectromechanical systems (MEMS). Here, a class of arsonic acid self‐assembled monolayers (SAMs) is reported that form readily on oxide substrates including silicon oxide, borosilicate glass, and titanium oxide. Monolayers are easily prepared using a straightforward soaking technique, which is amenable to large‐scale commercial applications. Monolayer formation on borosilicate glass and titanium oxide is characterized using infrared spectroscopy. Monolayers on borosilicate glass, native silicon oxide and titanium oxide are evaluated with contact angle measurements, as well as wear measurements using nanoscratching experiments. On titanium oxide and borosilicate glass, monolayers prepared from hexadecylarsonic acid provide significantly greater surface protection than surfaces reacted under similar conditions with hexadecylphosphonic acid, a common modifying agent for oxide substrates.     

Controlling Electron and Hole Charge Injection in Ambipolar Organic Field‐Effect Transistors by Self‐Assembled Monolayers

Controlling contact resistance in organic field‐effect transistors (OFETs) is one of the major hurdles to achieve transistor scaling and dimensional reduction. In particular in the context of ambipolar and/or light‐emitting OFETs it is a difficult challenge to obtain efficient injection of both electrons and holes from one injecting electrode such as gold since organic semiconductors have intrinsically large band gaps resulting in significant injection barrier heights for at least one type of carrier. Here, systematic control of electron and hole contact resistance in poly(9,9‐di‐n‐octylfluorene‐alt‐benzothiadiazole) ambipolar OFETs using thiol‐based self‐assembled monolayers (SAMs) is demonstrated. In contrast to common believe, it is found that for a certain SAM the injection of both electrons and holes can be improved. This simultaneous enhancement of electron and hole injection cannot be explained by SAM‐induced work‐function modifications because the surface dipole induced by the SAM on the metal surface lowers the injection barrier only for one type of carrier, but increases it for the other. These investigations reveal that other key factors also affect contact resistance, including i) interfacial tunneling through the SAM, ii) SAM‐induced modifications of interface morphology, and iii) the interface electronic structure. Of particular importance for top‐gate OFET geometry is iv) the active polymer layer thickness that dominates the electrode/polymer contact resistance. Therefore, a consistent explanation of how SAM electrode modification is able to improve both electron and hole injection in ambipolar OFETs requires considering all mentioned factors.     

Self‐Assembled Monolayers on Gold for the Fabrication of Radioactive Stents

An innovative and easily applicable method for the fabrication of radioactive stents, to be used for the treatment of restenosis, is presented. By incorporating the β‐emitting radioisotopes ¹⁸⁶Re, ¹⁸⁸Re, ⁹⁰Y, or ³²P into sulfur‐containing adsorbates, it becomes possible to cover a gold surface with a radioactive self‐assembled monolayer (SAM). Two methods have been investigated. In the first, SAMs consisting of potentially radioactive rhenium‐, yttrium‐, and phosphorus‐containing adsorbates have been assembled on 2D gold substrates, after which they have been studied by wettability measurements, electrochemistry, and X‐ray photoelectron spectroscopy (XPS). The stability of these SAMs under simulated physiological conditions (phosphate buffered saline, PBS solution) for periods up to two months has been demonstrated. Alternatively, potentially radioactive monolayers have been prepared by exposure of SAMs of mono‐, bi‐, and tridentate ligands to a solution containing a radiometal (rhenium) in order to bind the metal to the monolayer. The polydentate ligands exhibit excellent binding capacity, leading to SAMs containing over 10^–10 mol/cm² of the radiometal, which is more than sufficient to make this system viable for the delivery of therapeutical dosages of radiation.     

Bottom‐Up Engineering of Subnanometer Copper Diffusion Barriers Using NH2‐Derived Self‐Assembled Monolayers

A 3‐aminopropyltrimethoxysilane‐derived self‐assembled monolayer (NH₂SAM) is investigated as a barrier against copper diffusion for application in back‐end‐of‐line (BEOL) technology. The essential characteristics studied include thermal stability to BEOL processing, inhibition of copper diffusion, and adhesion to both the underlying SiO₂ dielectric substrate and the Cu over‐layer. Time‐of‐flight secondary ion mass spectrometry and X‐ray spectroscopy (XPS) analysis reveal that the copper over‐layer closes at 1–2‐nm thickness, comparable with the 1.3‐nm closure of state‐of‐the‐art Ta/TaN Cu diffusion barriers. That the NH₂SAM remains intact upon Cu deposition and subsequent annealing is unambiguously revealed by energy‐filtered transmission electron microscopy supported by XPS. The SAM forms a well‐defined carbon‐rich interface with the Cu over‐layer and electron energy loss spectroscopy shows no evidence of Cu penetration into the SAM. Interestingly, the adhesion of the Cu/NH₂SAM/SiO₂ system increases with annealing temperature up to 7.2 J m^?2 at 400 °C, comparable to Ta/TaN (7.5 J m^?2 at room temperature). The corresponding fracture analysis shows that when failure does occur it is located at the Cu/SAM interface. Overall, these results demonstrate that NH₂SAM is a suitable candidate for subnanometer‐scale diffusion barrier application in a selective coating for copper advanced interconnects.     

Contact Angle Analysis During the Electro‐oxidation of Self‐Assembled Monolayers Formed by n‐Octadecyltrichlorosilane

The electrochemical oxidation process of self‐assembled monolayers formed by n‐octadecyltrichlorosilane (OTS) molecules on silicon wafers has been studied in a droplet of water by means of in situ water contact angle measurements. The application of different bias voltages between the substrate and a counter electrode placed into the droplet resulted in changes of the chemical nature of the monolayer, which yielded a significant alteration of the surfaces properties. Due to the changes of the wetting properties of the monolayer during the electro‐oxidation process a change in the contact angles of the water droplet is concomitantly observed. This allows the in situ monitoring of the electro‐oxidation process for large modified areas of several millimeters in diameter. The chosen approach represents an easy way to screen the major parameters that influence the oxidation process. Afterwards, the oxidized regions are characterized by Fourier‐transform infrared (FT‐IR) spectroscopy, X‐ray photoelectron spectroscopy (XPS) measurements, and atomic force microscopy (AFM) investigations to obtain more information about the electro‐oxidation process. The observations are correlated to experimental results obtained for oxidations performed on a smaller dimension range in the water meniscus of a conductive, biased AFM tip. A good correlation of the results in the different dimension ranges could be found.     

Oriented Growth of Self‐Assembled Polyaniline Nanowire Arrays Using a Novel Method

A novel method for fabrication of highly oriented polyaniline (PANI) nanowires without removal of the template was developed by combining self‐assembly and template synthesis techniques. By using a self‐assembly process under inhibition conditions, oriented arrays of PANI nanowires growing out of the nanoporous template were obtained, with nanowire diameters ranging from 110 to 190 nm and lengths of several micrometers. The lengths of these wires can be roughly controlled by the polymerization time.     

Systematic Doping Control of CVD Graphene Transistors with Functionalized Aromatic Self‐Assembled Monolayers

Recent reports have shown that self‐assembled monolayers (SAMs) can induce doping effects in graphene transistors. However, a lack of understanding persists surrounding the quantitative relationship between SAM molecular design and its effects on graphene. In order to facilitate the fabrication of next‐generation graphene‐based devices it is important to reliably and predictably control the properties of graphene without negatively impacting its intrinsic high performance. In this study, SAMs with varying dipole magnitudes/directions are utilized and these values are directly correlated to changes in performance seen in graphene transistors. It is found that, by knowing the z‐component of the SAM dipole, one can reliably predict the shift in graphene charge neutrality point after taking into account the influence of the metal electrodes (which also play a role in doping graphene). This relationship is verified through density functional theory and comprehensive device studies utilizing atomic force microscopy, X‐ray photoelectron spectroscopy, Raman spectroscopy, and electrical characterization of graphene transistors. It is shown that properties of graphene transistors can be predictably controlled with SAMs when considering the total doping environment. Additionally, it is found that methylthio‐terminated SAMs strongly interact with graphene allowing for a cleaner graphene transfer and enhanced charge mobility.     

The Dielectric Constant of Self‐Assembled Monolayers

Self‐assembled monolayers (SAMs) are fundamental building blocks of molecular electronics and find numerous applications in organic (opto)electronic devices. Their properties are decisively determined by their response to electric fields, which are either applied externally (e.g., when biasing devices) or originate from within the monolayer itself in case it consists of dipolar molecules (which are used to tune charge‐injection barriers). This response is typically described by the dielectric constant of the monolayer. In this work it is explicitly show that there is no “general” dielectric constant that simultaneously applies to both cases. This is first derived on the basis of density‐functional theory (DFT) calculations for substituted biphenyl‐thiol SAMs at varying packing densities. Depolarization effects, which play a crucial role for the dielectric properties of the monolayers, are subsequently analyzed on the basis of packing‐dependent charge rearrangements. Finally, the DFT results are rationalized using an electrostatic model. In this context, the importance of finite‐size effects is highlighted and a connection between the macroscopic dielectric properties and the molecular polarizability is established providing a monolayer equivalent to the Clausius–Mossotti relationship. This allows deriving general trends for the packing‐density dependent dielectric response of monolayers to both external and internal electric fields.     

A Non‐Covalent Approach for Depositing Spatially Selective Materials on Surfaces

We describe a new method for depositing patterned materials, based on non‐covalent trapping of ligands in solvent‐templated nanocavities created in aromatic, self‐assembled monolayer or polymer films. A model has been developed and tested to describe nanocavity formation and the ligand adsorption process, which occurs via ligand exclusion from ambient, aqueous solution into the hydrophobic nanocavities. Ligand adsorption rates and ligand adsorbate reactivity with solution species are governed by ligand size/geometry design factors identified using the model. Spatial control of adsorption is achieved via film photochemical changes that inhibit subsequent ligand adsorption/accessibility (UV or X‐ray) or displacement of entrapped ligands (50 keV electron‐beam) during film patterning. The reactivity of the adsorbed ligand is illustrated by the selective binding of Pd^II species that catalyze electroless metal deposition. Fabrication of high‐resolution (≈ 50 nm), positive‐tone patterns in nickel with acceptable feature‐edge acuity and critical dimension control (≈ 5 %) is demonstrated.