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Structure and Growth of Self-Assembling Monolayers
Abstract
The structural phases and the growth of self-assembled monolayers (SAMs) are reviewed from a surface science perspective, with emphasis on simple model systems. The concept of self-assembly is explained, and dierent self-assembling materials are brie¯y discussed. A summary of the techniques used for the study of SAMs is given. Dierent general scenarios for structures obtained by self-assembly are described. Thiols on Au(1 1 1) surfaces are used as an archetypal system to investigate in detail the structural phase diagram as a function of tem-perature and coverage, the speci®c structural features on a molecular level, and the eect of changes of the molecular backbone and the end group on the structure of the SAM. Tem-perature eects including phase transitions are discussed. Concepts for the preparation of more complex structures such as multi-component SAMs, laterally structured SAMs, and heterostructures, also with inorganic materials, are outlined. The growth and ways to control it are discussed in detail. Solution and gas phase deposition and the impact of various pa-rameters such as temperature, concentration (in solution) or partial pressure (in the gas phase) are described. The kinetics and the energetics of self-assembly are analyzed. Several more complex issues of the ®lm formation process including non-equilibrium issues are discussed. Some general conclusions are drawn concerning the impact of various molecular features on the growth behavior and concerning the relationship between growth and structural phase diagram. Finally, the potential of self-assembly as a route for the preparation of monolayers with pre-designed properties and SAMs as building blocks in heterostructures as well as ap-plication strategies are discussed.
... They comprise a carbazole functional body with a phosphonic acid anchoring group. 7,28 Substantial differences in crystal growth of co-evaporated perovskites were observed, depending on whether or not MeO-2PACz was washed with ethanol to remove residual bulk SAM material before the deposition. 24 As the difference in contact angle for washed and unwashed MeO-2PACz is insignificant, as shown in our previous work, 29 the authors postulated that the difference in growth can be attributed to the formation of hydrogen bonds between exposed phosphonic acid functional groups and formamidinium iodide (FAI) during the deposition process. ...
... As a more general mechanism, p-p interactions between carbazole functional groups are substituent dependent, 36,37 causing the expected density of ''bilayer'' nPACz materials with exposed phosphonic functional groups to differ between our materials. This is coupled with the expected orientation of phosphonic acid functional groups to become non-uniform at low coverages due to the potential for tilted SAM configurations, 28 which are more likely with increasing alkyl chain length. Changes in this density of exposed phosphonic acids may also explain the differences in relative FAI rate required for the highest-performing devices. ...
Interfacial engineering has fueled recent development of p-i-n perovskite solar cells (PSCs), with self-assembled monolayer-based hole-transport layers (SAM-HTLs) enabling almost lossless contacts for solution-processed PSCs, resulting in the highest achieved power conversion efficiency (PCE) to date. Substrate interfaces are particularly crucial for the growth and quality of co-evaporated PSCs. However, adoption of SAM-HTLs for co-evaporated perovskite absorbers is complicated by the under explored interaction of such perovskites with phosphonic acid functional groups. In this work, we highlight how exposed phosphonic acid functional groups impact the initial phase and final bulk crystal structures of co-evaporated perovskites and their resultant PCE. The explored surface interaction is mediated by hydrogen bonding with interfacial iodine,leading to increased formamidinium iodide adsorption, persistent changes in perovskite structure, and stabilization of bulk a-FAPbI3, hypothesized as being due to kinetic trapping. Our results highlight the potential of exploiting substrates to increase control of co-evaporated perovskite growth. © 2024 Elsevier Inc.
... The widespread miniaturization of electronic devices, the introduction of new biochemical analysis techniques, and developments in medicine, modern textile production, and information technology require materials and systems with micro-or nanometer dimensions. The consequence of this trend is the development of new modification and production methods for micro-and nanomaterials and the use of modern techniques to characterize objects at the micro-and nanoscale [1][2][3][4]. An effect of miniaturization in industry is also the development of micromechanical systems (MEMSs). ...
... These parameters have a significant impact on the tribological behavior of the modified surfaces. The higher packing density and more ordered SAM layers exhibited by surfaces consisting of organosilane compounds with alkyl chains of at least eight carbon atoms are the result of stronger van der Waals interactions between silane molecules [1,46]. Higher coefficients of friction for self-assembled monolayers consisting of short-chain alkylsilanes or arylosilanes (e.g., PTCS) can be explained by the fact that, during the formation of such SAMs, horizontal polymerization occurs at the same time as vertical polymerization (oligomerization). ...
In this work, Ti-incorporated carbon coatings were used as substrates for modification with one- and two-component self-assembled monolayers of organosilane compounds using a polydimethylsiloxane (PDMS) stamp. This enabled the selective functionalization of surfaces with micrometric dimensions. The topography of the modified surfaces was defined using an atomic force microscope (AFM). The effectiveness of the modification was confirmed by measurements of the water contact angle and surface free energy using the Oss and Good method. Using a T-23 microtribometer with counterparts in the shape of balls that were made of steel, silicon nitride (Si3N4), and zirconium dioxide (ZrO2), the tribological properties of the obtained coatings were tested. These investigations showed that modification by using a PDMS stamp makes it possible to produce two-component ultrathin silane layers on Ti-containing carbon substrates. Two-component organosilane layers had higher hydrophobicity, a lower friction coefficient, and a smaller width of wear tracks than the one-component analogs. It was also found that the work of adhesion of the created surfaces had a significant influence on the value of the friction coefficient and the percentage value of the growth inhibition of bacteria.
... To implement the FcC6SH memory cell, one can imagine the cation anchored on a gold substrate through S-Au covalent bonding, e.g. by creating a Self-Assembled Monolayers [29], and we hypothesize the application of electric fields force the molecule to bend on the substrate. The binary information corresponds thus to the FcC6SH bending direction. ...
... Notice that, in this calculation, the substrate is emulated and not explicitly considered. It is possible to verify experimentally that the FcC6SH generally lies on the substrate [29]. Therefore, if the molecule is exposed to the electric field, the substrate accentuates the bending. ...
... The packing densities were evaluated on the basis of the S 2p/Au 4f intensity ratio, assuming similar attenuation of the respective signals. Literature values of the attenuation lengths of the photoemission signals at the relevant kinetic energies were used [45]. The spectrometer-specific coefficients were determined by using a reference C16 SAM with well-known thickness (1.89 ± 0.02 nm) and packing density (4.63 × 10 14 molecules/cm 2 ) [46]. ...
... It is possible that this kind of adaption also takes place to some extent in the current system, as the PFCH groups in both monolayers are significantly canted within the ring planes, presumably to maximize the intermolecular ring interactions parallel to the surface (dipolar interactions and H … F bridges, Fig. 10). Consequently, the odd-even effects are noticeably reduced in the PFCH-n SAMs, but are still well perceptible in the packing density, molecular orientation, and the stability against exchange, with the right relation to the parity of the methylene group number in the alkyl linker (see Refs. [45][46][47][48][49]). In contrast, these effects are fully compensated in the wetting behavior, which is basically the same for both layers. ...
All- cis -hexafluoro- and all-cis-pentafluoro-cyclohexane (PFCH) derivatives are new kinds of materials, the structures and properties of which are dominated by the highly dipolar Janus-face motif. Here, we report on the effects of integrating the PFCH groups into self-assembled monolayers (SAMs) of alkanethiolates on Au(111). Monolayers with an odd (eleven) and even (twelve) number of methylene groups were characterized in detail by several complementary experimental tools, supported by theoretical calculations. Surprisingly, all the data show a high similarity of both kinds of monolayers, nearly lacking the typically observed odd-even effects. These new monolayers have a packing density about 1/3 lower than that of non-substituted alkanethiolate monolayers, caused by the bulkiness of the PFCH moieties. The orientations of the PFCH groups and the alkyl chains could be determined independently, suggesting a conformation similar to the one found in the solid state structure of an analogous compound. Although in the SAMs the PFCH groups are slightly tilted away from the surface normal with the axial fluorine atoms pointing downwards, most of the dipole moments of the group remain oriented parallel to the surface, which is a unique feature for a SAM system. The consequences are much lower water contact angles compared to other partly fluorinated SAMs as well as rather moderate work function values. The interaction between the terminal PFCH moieties results in an enhanced stability of the PFCH-decorated SAMs toward exchange reaction with potential molecular substituents in spite of the lower packing density of these films.
... They act as both a physical and chemical barrier to block any subsequent deposition on this area while still allowing growth on other areas or materials on the surface [7]. SAMs are compact organic monomolecular layers that spontaneously adsorb on a surface, showing large-scale ordering via Van der Waals force once deposited [24]. SAMs are comprised of a head group with a strong affinity for the substrate, a backbone chain, and a terminal functional group. ...
Perfluorododecyl iodide (I-PFC12) is of interest for area-selective deposition (ASD) applications as it exhibits intriguing properties such as ultralow surface energy, the ability to modify silicon’s band gap, low surface friction, and suitability for micro-contact patterning. Traditional photolithography is struggling to reach the required critical dimensions. This study investigates the potential of using I-PFC12 as a way to produce contrast between the growth area and non-growth areas of a surface subsequent to extreme ultraviolet (EUV) exposure. Once exposed to EUV, the I-PFC12 molecule should degrade with the help of the photocatalytic substrate, allowing for the subsequent selective deposition of the hard mask. The stability of a vapor-deposited I-PFC12 self-assembled monolayer (SAM) was examined when exposed to ambient light for extended periods of time by using X-ray photoelectron spectroscopy (XPS). Two substrates, SiO2 and TiO2, are investigated to ascertain the suitability of using TiO2 as a photocatalytic active substrate. Following one month of exposure to light, the atomic concentrations showed a more substantial fluorine loss of 10.2% on the TiO2 in comparison to a 6.2% loss on the SiO2 substrate. This more pronounced defluorination seen on the TiO2 is attributed to its photocatalytic nature. Interestingly, different routes to degradation were observed for each substrate. Reference samples preserved in dark conditions with no light exposure for up to three months show little degradation on the SiO2 substrate, while no change is observed on the TiO2 substrate. The results reveal that the I-PFC12 SAM is an ideal candidate for resistless EUV lithography.
... nm 2 ) and the structure of n-alkyl thiols on gold which has been studied and described as an hexagonal √3 × √3R 30°arrangement of thiol molecules with a sulfur−sulfur distance of around 5 Å and an area per molecules of 21.6 Å 2 . 48,49 Moreover, based on the HPAEC-PAD results, the mannose-PEG grafting density was calculated and found to be around 4 molecules·nm −2 (calculation in the Supporting Information). This is in agreement with the values reported in the literature in which for PEG ligands on spherical gold nanoparticles a grafting density of 3.5 molecules·nm −2 was obtained by quantitative proton nuclear magnetic resonance and thermogravimetric analysis, 50 and a surface coverage from 4.3 to 6.3 molecules· nm −2 by inductively coupled plasma mass spectrometry. ...
Mannose-binding lectin (MBL) activates the complement system lectin pathway and subsequent inflammatory mechanisms. The incidence and outcome of many human diseases, such as brain ischemia and infections, are associated with and influenced by the activity and serum concentrations of MBL in body fluids. To quantify MBL levels, tests based on ELISA are used, requiring several incubation and washing steps and lengthy turnaround times. Here, we aimed to develop a nanoplasmonic assay for direct MBL detection in human serum at the point of care. Our assay is based on gold nanorods (GNRs) functionalized with mannose (Man-GNRs) via an amphiphilic linker. We experimentally determined the effective amount of sugar linked to the nanorods’ surface, resulting in an approximate grafting density of 4 molecules per nm², and an average number of 11 to 13 MBL molecules binding to a single nanoparticle. The optimal Man-GNRs concentration to achieve the highest sensitivity in MBL detection was 15 μg·mL–1. The specificity of the assay for MBL detection both in simple buffer and in complex pooled human sera was confirmed. Our label-free biosensor is able to detect MBL concentrations as low as 160 ng·mL–1 within 15 min directly in human serum via a one-step reaction and by using a microplate reader. Hence, it forms the basis for a fast, noninvasive, point-of-care assay for diagnostic indications and monitoring of disease and therapy.
... Self-assembled monolayers (SAMs) begin to form when molecules from vapor or liquid interact and strongly connect with the substrate surface via head groups [72][73][74][75] , typically thiols, silanes, and phosphonates. Chemical bonds are formed between the head groups and the substrate. ...
E-beam lithography is a powerful tool for generating nanostructures and fabricating nanodevices with fine features approaching a few nanometers in size. However, alternative approaches to conventional spin coating and development processes are required to optimize the lithography procedure on irregular surfaces. In this review, we summarize the state of the art in nanofabrication on irregular substrates using e-beam lithography. To overcome these challenges, unconventional methods have been developed. For instance, polymeric and nonpolymeric materials can be sprayed or evaporated to form uniform layers of electron-sensitive materials on irregular substrates. Moreover, chemical bonds can be applied to help form polymer brushes or self-assembled monolayers on these surfaces. In addition, thermal oxides can serve as resists, as the etching rate in solution changes after e-beam exposure. Furthermore, e-beam lithography tools can be combined with cryostages, evaporation systems, and metal deposition chambers for sample development and lift-off while maintaining low temperatures. Metallic nanopyramids can be fabricated on an AFM tip by utilizing ice as a positive resistor. Additionally, Ti/Au caps can be patterned around a carbon nanotube. Moreover, 3D nanostructures can be formed on irregular surfaces by exposing layers of anisole on organic ice surfaces with a focused e-beam. These advances in e-beam lithography on irregular substrates, including uniform film coating, instrumentation improvement, and new pattern transferring method development, substantially extend its capabilities in the fabrication and application of nanoscale structures.
... A capacidade que moléculas auto-organizáveis têm de adsorver e formar uma estrutura organizada em uma superfície metálica ativa, tais como ferro, alumínio e zinco, tem atraído o interesse tanto como pré-tratamento de materiais metálicos para aumentar resistência à corrosão, como para outras aplicações, destacando-se, por exemplo, a construção de biossensores [10] e sensores eletroquímicos [11]. O processo de formação de uma camada auto-organizável ocorre espontaneamente pela imersão do substrato metálico em uma solução de moléculas surfactantes e, com a adsorção, forma-se uma monocamada altamente organizada, com estrutura densa e estável [12]. Compostos derivados de alcanos com um ou dois grupos funcionais são bastante usados para essa finalidade. ...
Tratamentos de superfície baseados em deposição de
filmes finos são técnicas frequentemente utilizadas
para aumentar a resistência à corrosão de ligas de
alumínio. O uso de moléculas que adsorvem na superfície
formando uma estrutura auto-organizada tem se apresentado como uma alternativa viável para esta finalidade. Todavia, as variáveis e condições adotadas na deposição destes
filmes podem influenciar o desempenho desse revestimento.
No presente estudo, espectroscopia de impedância eletroquímica e curvas de polarização potenciodinâmica revelaram
que o tratamento com moléculas auto-organizáveis (SAM)
melhora a resistência à corrosão da liga 5052, porém, é
muito importante o pré-tratamento da superfície antes do
tratamento com SAM. Os resultados obtidos foram comparados com os de tratamentos de passivação com Cr(III) e
Cr(VI) e indicaram melhores resistências à corrosão para a
liga tratada com moléculas auto-organizáveis frente a esses
dois tratamentos de passivação com cromo. Estes resultados sugerem a possibilidade de substituição de tratamentos
que geram produtos tóxicos como o Cr(VI) por tratamentos ambientalmente amigáveis.
... The area of the nanopore created by the removal of the melamine unit has been estimated within the Spartan suit of programs as 0.37 nm 2 , whilst the area of a thiol compound is ≈ 0.22 nm 2 . [30] This result may tentatively be interpreted under the assumption that the dendritic units retract upon the rupture of hydrogen bonds with the melamine unit. ...
Attaining precise control over molecular arrangements is of paramount importance for numerous applications in nanotechnology, particularly in constructing molecular templates to accurately immobilize target materials on surfaces. A strategic combination of supramolecular and interfacial chemistry may serve to build a well‐organized molecular network, enabling the subsequent location of target molecules on specific positions of a surface. A supramolecular complex (compound 1) comprised of a melamine unit forming hydrogen bonds with dendritic arms terminated in a coumarin unit is utilized, which readily undergoes photodimerization. The research demonstrates the formation of well‐organized Langmuir films of compound 1 which can be transferred on substrates at low surface pressures adopting a lying‐flat orientation. Upon irradiation of the pristine films at 365 nm the coumarin units undergo photo‐cross linking, leading to the formation of a compact photo‐crosslinked film. Incubation of these photo‐crosslinked films in a solution containing 1‐hexanethiol results in the withdrawal of the melamine and the chemisorption of two thiol molecules per each melamine unit. The nanopores created by the removal of the melamine core are attributed to the disruption of hydrogen bonds in compound 1 by the thiols. This precisely defined molecular network holds significant promise as a template for orchestrating the arrangement of functional materials on surfaces.
... It has been demonstrated that the adsorption of organic disulfides or organic thiols on gold surfaces occurs via the cleavage of the S-S bonds of organic disulfide or the cleavage of the S-H bond in organic thiols, resulting in the formation of identical gold-bound thiolate SAMs [39][40][41]. Therefore, it was reasonable to consider that the adsorption of DPymDS on Au(111) results in the formation of 2PymS SAMs, as shown in the schematic view in Figure 1. ...
The effects of solution concentration and pH on the formation and surface structure of 2-pyrimidinethiolate (2PymS) self-assembled monolayers (SAMs) on Au(111) via the adsorption of 2,2′-dipyrimidyl disulfide (DPymDS) were examined using scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS). STM observations revealed that the formation and structural order of 2PymS SAMs were markedly influenced by the solution concentration and pH. 2PymS SAMs formed in a 0.01 mM ethanol solution were mainly composed of a more uniform and ordered phase compared with those formed in 0.001 mM or 1 mM solutions. SAMs formed in a 0.01 mM solution at pH 2 were composed of a fully disordered phase with many irregular and bright aggregates, whereas SAMs formed at pH 7 had small ordered domains and many bright islands. As the solution pH increased from pH 7 to pH 12, the surface morphology of 2PymS SAMs remarkably changed from small ordered domains to large ordered domains, which can be described as a (4√2 × 3)R51° packing structure. XPS measurements clearly showed that the adsorption of DPymDS on Au(111) resulted in the formation of 2PymS (thiolate) SAMs via the cleavage of the disulfide (S-S) bond in DPymDS, and most N atoms in the pyrimidine rings existed in the deprotonated form. The results herein will provide a new insight into the molecular self-assembly behaviors and adsorption structures of DPymDS molecules on Au(111) depending on solution concentration and pH.
... Other methods for edge lithography, such as edge-spreading lithography (ESL) and secondary sputtering lithography (SSL), have also been reported. ESL utilizes the fluidity of the target material to create structures and is intended for self-assembled monolayers (SAMs) [16,17]. A relief structure like SiO 2 beads [18] or photoresist lines [19] is first created on the substrate. ...
... This is a reasonable number, taking into account the relative weight of the thiolate signal (∼47%) and the ultimate packing density for the tripodal-anchored, triptycene-based SAM (4.6 × 10 14 thiolates/cm 2 ). 13 The second important parameter, determined on the basis of the C 1s/Au 4f intensity ratio and, once again, the C16 SAM (1.89 nm) 35 as a reference, is the effective thickness. This parameter was estimated at ∼1.62 nm, which is quite close to the 'height' of the properly anchored Fc-Trip molecule (1.67 nm; see Figure 1). ...
... (4) O processo de formação de monocamadas com alto grau de organização estrutural ocorre espontaneamente sobre a superfície de um substrato metálico por adsorção das moléculas surfactantes em solução. (6) A deposição de filmes com espessura nanométrica vem sendo amplamente estudada devido as suas aplicações em diferentes campos, como a química, física e biologia. Em filmes finos é possível proporcionar organização molecular, controlar a interação desses materiais de uma forma refinada, inclusive no nível molecular. ...
... Secondly, the molecular weight determines the adsorption type, generally, the small molecules easily form the chemisorption on the metal surface, the large molecules tend to the physisorption on the metal surface. Because the chemisorption exhibits short-range interaction, and physisorption is the interaction of van der Waals forces, which is long-range interaction [37]. ...
... [17][18][19] SAMs with stable, densely packed, and well-defined structures can be achieved from either the solution or the gas phase. [20][21][22][23][24] Such analyte-specific SAM-based sensors require the incorporation of the appropriate receptor. A wide variety of selective receptors based on host-guest chemistry include cyclodextrins (CDs), [25][26][27] crown ethers, [28,29] calixarenes, [30,31] cucurbit[n]urils (CB[n]s), [32] pillarenes, [33] and bambus [6]urils (BU [6]s). ...
In the last two decades, perchlorate salts have been identified as environmental pollutants and recognized as potential substances affecting human health. We describe self‐assembled monolayers (SAMs) of novel semiaza‐bambus[6]urils (semiaza‐BUs) equipped with thioethers or disulfide (dithiolane) functionalities as surface‐anchoring groups on gold electrodes. Cyclic voltammetry (CV) with Fe(CN)63−/4− as a redox probe, together with X‐ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and ellipsometry, were employed to characterize the interactions at the interface between the anchoring groups and the metal substrate. Data showed that the anion receptors′ packing on the gold strongly depends on the anchoring group. As a result, SAMs of BUs with lipoic amide side chains show a concentration‐dependent layer thickness. The BU SAMs are extremely stable on repeated electrochemical potential scans and can selectively recognize perchlorate anions. Our electrochemical impedance spectroscopy (EIS) studies indicated that semiaza‐BU equipped with the lipoic amide side chains binds perchlorate (2–100 mM) preferentially over other anions such as F⁻, Cl⁻, I⁻, AcO⁻, H2PO4⁻, HPO4²⁻, SO4²⁻, NO2⁻, NO3⁻, or CO3²⁻. The resistance performance is 10 to 100 times more efficient than SAMs containing all other tested anions.
... One prominent example is soft lithography, a surface-patterning technique that uses various printing methods to deposit SAM patterns on surfaces. This approach bridged the gap between physical chemistry and nanolithography, providing a simple and reachable approach to construct nanostructures on a chemical laboratory bench [19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37]. ...
Water contamination has become a global crisis, affecting millions of people worldwide and causing diseases and illnesses, including cholera, typhoid, and hepatitis A. Conventional water remediation methods have several challenges, including their inability to remove emerging contaminants and their high cost and environmental impact. Nanomembranes offer a promising solution to these challenges. Nanomembranes are thin, selectively permeable membranes that can remove contaminants from water based on size, charge, and other properties. They offer several advantages over conventional methods, including their ability to remove evolving pollutants, low functioning price, and reduced ecological influence. However, there are numerous limitations linked with the applications of nanomembranes in water remediation, including fouling and scaling, cost-effectiveness, and potential environmental impact. Researchers are working to reduce the cost of nanomembranes through the development of more cost-effective manufacturing methods and the use of alternative materials such as graphene. Additionally, there are concerns about the release of nanomaterials into the environment during the manufacturing and disposal of the membranes, and further research is needed to understand their potential impact. Despite these challenges, nanomembranes offer a promising solution for the global water crisis and could have a significant impact on public health and the environment. The current article delivers an overview on the exploitation of various engineered nanoscale substances, encompassing the carbonaceous nanomaterials, metallic, metal oxide and metal–organic frameworks, polymeric nano-adsorbents and nanomembranes, for water remediation. The article emphasizes the mechanisms involved in adsorption and nanomembrane filtration. Additionally, the authors aim to deliver an all-inclusive review on the chronology, technical execution, challenges, restrictions, reusability, and future prospects of these nanomaterials.
... The distance between supports was 35 mm (the norm indicating a space of 33 mm). The ultimate load, F max , was considered as the failure initiation measurement [22]. The bending stress was calculated by dividing the load by the adhesive surface area. ...
The objective of this study is to replace steel-phosphating treatments with self-assembled monolayers (SAMs), in this case, of phosphonic acids. Two industrial applications are targeted: obtaining an adhesion primer between steel and paint or lubricating stainless steel for mechanical applications, such as stamping; research has been conducted using alkyl phosphonic acids. The carbon chain length and terminal function are changed to obtain the best properties depending on the application.
... SAM is a 2-dimensional molecular assemblies formed on a solid surface by the adsorption and organization of an active surfactant. The driving force for spontaneous SAM formation is the chemical bond synthesis by intermolecular interactions between the substrate surface and surfactant molecules [51,52]. ...
Nanoimprint lithography (NIL) has attracted attention recently as a promising fabrication method for dielectric metalenses owing to its low cost and high throughput, however, high aspect ratio (HAR) nanostructures are required to manipulate the full 2π phase of light. Conventional NIL using a hard-polydimethylsiloxane (h-PDMS) mold inevitably incurs shear stress on the nanostructures which is inversely proportional to the surface area parallel to the direction of detachment. Therefore, HAR structures are subjected to larger shear stresses, causing structural failure. Herein, we propose a novel wet etching NIL method with no detachment process to fabricate flawless HAR metalenses. The water-soluble replica mold is fabricated with polyvinyl alcohol (PVA) which is simpler than an h-PDMS mold, and the flexibility of the PVA mold is suitable for direct printing as its high tensile modulus allows high-resolution patterning of HAR metalenses. The diffraction-limited focusing of the printed metalenses demonstrates that it operates as an ideal lens in the visible regime. This method can potentially be used for manufacturing various nanophotonic devices that require HAR nanostructures at low cost and high throughput, facilitating commercialization.
... Polymers can also form ultrathin functional layers by depositing short polymer chains with functional end groups on surfaces, where they form so-called polymer brushes. [15][16][17][18] They usually tether to the surfaces by physisorption or covalent chemical bonding of the end groups. Whereas the alkane chains of SAM-forming molecules arrange themselves into ordered layers of defined thickness by interaction with each other, polymer brushes consist of an amorphous layer of polymer chains, where chain length and coverage density determine the layer thickness. ...
Patterned, ultra‐thin surface layers can serve as templates for positioning nanoparticlesor targeted self‐assembly of molecular structures, for example, block‐copolymers. This work investigates the high‐resolution, atomic force microscopebased patterning of 2 nm thick vinyl‐terminated polystyrene brush layers and evaluates the line broadening due to tip degradation. This work compares the patterning properties with those of a silane‐based fluorinated self‐assembled monolayer (SAM), using molecular heteropatterns generated by modified polymer blend lithography (brush/SAM‐PBL). Stable line widths of 20 nm (FWHM) over lengths of over 20000 µm indicate greatly reduced tip wear, compared to expectations on uncoated SiOx surfaces. The polymer brush acts as a molecularly thin lubricating layer, thus enabling a 5000 fold increase in tip lifetime, and the brush is bonded weakly enough that it can be removed with surgical accuracy. On traditionally used SAMs, either the tip wear is very high or the molecules are not completely removed. Polymer Phase Amplified Brush Editing is presented, which uses directed self‐assembly to amplify the aspect ratio of the molecular structures by a factor of 4. The structures thus amplified allow transfer into silicon/metal heterostructures, fabricating 30 nm deep, all‐silicon diffraction gratings that could withstand focused high‐power 405 nm laser irradiation.
... The 2D structure results from maximum material interaction with water, forming a very thin polymeric monolayer, as seen in Figure 4. The phase of polymeric Langmuir film can be divided into extended and condensed stages [12]. An expanded monolayer is a 2D "dissolved" state dependent upon hydration forces with the subphase, whereas a condensed monolayer is a 2D "collapsed" state. ...
Isotherms are generally curves that depict the phenomena that govern a substance's mobility at a constant temperature and pH. In this study, the Langmuir layer of P3OT, P3HT and PCBM were characterised by computing their surface pressure as a function of the surface area available to the molecules at the interface to obtain a curve called surface pressure-area (Π-A) isotherm. All three polymers were spread on two types of subphases-DI water and water containing bivalent metal ions, Pb2+. None of the Langmuir layers exhibits discrete gas-liquid-solid phase transitions on the water subphase. However, more stable Langmuir layers formed when lead ions were added to the water subphase. The stability enables the capping of lead ions between the polymer chain or within the balls, which can be implemented in flexible electronic devices.
The functional groups exhibit a dual influence on the covalent and non-covalent interactions between MBT and the Cu(111) surface via robust correlation with the Hammett substituent constant and substitution interaction energy, respectively.
Surface-specific sum frequency generation vibrational spectroscopy is applied to study the molecular configuration of short-chain n-alkanethiol self-assembled monolayers (SAMs with n = 2–6) on the Au surface. For monolayers with n≥ 3, the alkanethiols are upright-oriented, with the CH3 tilt angle varying between ∼33° and ∼46° in clear even–odd dependency. The ethanethiol monolayer (n = 2) is, however, found to exhibit a distinct lying-down configuration with a larger methyl tilt angle (67°–79°) and a smaller CH2 tilt angle (56°–68°). Such a unique configurational transition from n = 2 to n≥ 3 discloses the steric effect owing to chain–chain interaction among neighboring molecules. Through density functional theory calculations, the transition is further confirmed to be energetically favorable for thiols on a defective reconstructed Au(111) surface but not on the pristine one. Our study highlights the roles of the chain–chain interaction and the substrate surface atomic structure when organizing SAMs, offering a strategic pathway for exploiting their applications.
In metal‐halide perovskite solar cells (PSCs), various carrier recombination losses occur at the interface between metal oxides (MOs) and perovskite (PVK) due to the imperfect lattice structure of the crystal surface. Additionally, the nonoptimal energy levels of MOs and PVK, as well as ion diffusion and chemical corrosion between the two materials, severely hinder carrier transport at the interface. Therefore, there is an urgent need to introduce multifunctional materials between MOs and PVK to mitigate interface defects, carrier transport limitations, chemical corrosion, and other related issues. In recent years, self‐assembled monolayers (SAMs) have emerged as essential organic interfacial materials for effectively bridging MOs and PVK, playing a pivotal role in enhancing cells’ performance. Based on this, we provide a detailed overview of the origin and development of SAMs in PSCs and summarize the importance and potential of SAMs from various aspects, including their chemical structure, interface passivation, energy level tuning, and interface corrosion. We finally discuss the prospects of SAMs in terms of molecular structure, deposition methods, and their application in narrow‐band gap PSCs. With these insights, it is anticipated that SAMs will assist in realizing larger, highly efficient, stable, and cost‐effective PSCs, thereby enhancing the competitiveness of PSCs in the solar photovoltaics market.
Despite the common expectation that conjugated organic molecules on metals adsorb in a flat-lying layer, several recent studies have found coverage-dependent transitions to upright-standing phases, which exhibit notably different physical properties. In this work, we argue that from an energetic perspective, thermodynamically stable upright-standing phases may be more common than hitherto thought. However, for kinetic reasons, this phase may often not be observed experimentally. Using first-principles kinetic Monte Carlo simulations, we find that the structure with lower molecular density is (almost) always formed first, reminiscent of Ostwald’s rule of stages. The phase transitions to the upright-standing phase are likely to be kinetically hindered under the conditions typically used in surface science. The simulation results are experimentally confirmed for the adsorption of tetracyanoethylene on Cu(111) using infrared and X-ray photoemission spectroscopy. Investigating both the role of the growth conditions and the energetics of the interface, we find that the time for the phase transition is determined mostly by the deposition rate and, thus, is mostly independent of the nature of the molecule.
Polymer physics has evolved significantly over the past century, transitioning from the early recognition of the chain structure of polymers to a mature field integrating principles from statistical mechanics, thermodynamics, and condensed matter physics. As an important part of polymer physics, polymer single crystals are crucial for understanding molecular structures and behaviors, enhancing material properties, and enabling precise functionalization. They offer insights into polymer crystallization kinetics, serve as templates for nanofabrication, and have applications in electronics, sensors, and biomedical fields. However, due to the complexity of molecular chain movement, the formation of polymer single crystals is still very difficult. Over the decades, numerous researchers have dedicated themselves to unraveling the mysteries of polymer single crystals, yielding substantial findings. This paper focus on the historical evolution and advancements in polymer single crystal research, aiming to offer valuable insights and assistance to fellow researchers in this field.
Aryl-substituted alkanethiolate (AT) self-assembled monolayers (SAMs) exhibit typically so-called odd-even effects, viz. systematic variations in the film structure, packing density, and molecular inclination depending on the parity of the number of the methylene units in the alkyl linker, n. As an exception to this rule, ATs carrying an anthracen-2-yl group (Ant-n) as tail group were reported to have different behavior due the non-symmetric attachment of the anthracene unit to the AT linker, providing additional degree of freedom for the molecular organization and allowing for partial compensation of the odd-even effects. In this context, the structure of SAMs formed by adsorption of anthracene-substituted ATs (Ant-n; n = 1-6) at room temperature on Au(111) substrate was investigated by high-resolution scanning tunnelling microscopy (STM). Most of these SAMs exhibit a coexistence of two different ordered phases, some of which are common for either n = odd or n = even while other vary over the series, showing a broad variety of different structures. The average packing density of the Ant-n SAMs, derived from the analysis of the STM data, varies by 7.5-10% depending on the parity of n, being, as expected, higher for n = odd. The respective extent of the odd-even effects is noticeably lower than that usually observed for other aryl-substituted monolayers (∼25%), which goes in line with the previous findings and emphasizes the impact of the non-symmetric attachment of the aromatic unit.
Heterogeneous junctions extensively exist in electronic and photovoltaic devices. Due to essential differences, the contacts of heterogeneous junctions are imperfect with structural discontinuity and chemical inconsistency, which have negative impacts on the mechanical, electrical, and thermal properties of devices. To improve the heterogeneous interactions, surface/interfacial modification approaches are developed in which molecular assembly engineering appears to be a promising strategy. Versatile functionalities can be accomplished by smart arrangement and design of the functional groups and geometry of the organic molecular layers. Specific functionality can also be maximized by well organization of the grafting orientation of molecules at the heterogeneous contacts. This article comprehensively reviews the approaches of molecular assembly engineering employed in the construction of the heterogeneous junctions to improve their mechanical, electrical, and thermal properties. Following the introduction of molecular assembly engineering at the target surface/interface, examples are introduced to show the efficacy of molecular assembly engineering on the interfacial adhesion, atomic interdiffusion, dielectric nature, charge injection and recombination, and thermoelectric property in electronic and photovoltaic devices.
Self-assembled monolayers (SAMs) represent an important tool in context of nanofabrication and molecular engineering of surfaces and interfaces. The properties of functional SAMs depend not only on the character of the tail groups at the SAM-ambient interface, but are also largely defined by their structure. In its turn, the latter parameter results from a complex interplay of the structural forces and a variety of other factors, including so called odd-even effects, viz. dependence of the SAM structure and properties on the parity of the number (odd or even) of individual building blocks in the backbone of the SAM constituents. The most impressive manifestation of the odd-even effects is the structure of aryl-substituted alkanethiolate SAMs on Au(111) and Ag(111), in which, in spite of the fact that the intermolecular interaction is mostly determined by the aryl part of the monolayers, one observes a pronounced dependence of molecular inclination and, consequently, the packing density of the SAM-forming molecules on the parity of number of methylene units in the alkyl linker. Here we review the properties of the above systems as well as address fundamental reasons behind the odd-even effects, including the existence of a so-called bending potential, which is frequently disregarded in analysis of the structure-building forces. The generality of the odd-even effects in SAMs is additionally supported by the recent data for SAMs on GaAs, scanning tunneling microscopy data for SAMs on Ag(111), and the data for the monolayers with selenolate and carboxyl anchoring groups on Au(111) and Ag(111). The implications of these effects in terms of the control over the packing density and orientation of the tail groups at the SAM-ambient interface, structural perfection, polymorphism, temperature-driven phase transitions, and SAM stability toward such factors as ionizing radiation, exchange reaction, and electrochemical desorption are discussed. These implications place the odd-even effects as an important tool for the design of functional SAMs in context of specific applications.
In view of the relevance of organic thin layers in many fields, the fundamentals, growth mechanisms, and dynamics of thin organic layers, in particular thiol-based self-assembled monolayers (SAMs) on Au(111) are systematically elaborated. From both theoretical and practical perspectives, dynamical and structural features of the SAMs are of great intrigue. Scanning tunneling microscopy (STM) is a remarkably powerful technique employed in the characterization of SAMs. Numerous research examples of investigation about the structural and dynamical properties of SAMs using STM, sometimes combined with other techniques, are listed in the review. Advanced options to enhance the time resolution of STM are discussed. Additionally, we elaborate on the extremely diverse dynamics of various SAMs, such as phase transitions and structural changes at the molecular level. In brief, the current review is expected to supply a better understanding and novel insights regarding the dynamical events happening in organic SAMs and how to characterize these processes.
Organic/inorganic interfaces are known to exhibit rich polymorphism, where different polymorphs often possess significantly different properties. Which polymorph forms during an experiment depends strongly on environmental parameters such as deposition temperature and partial pressure of the molecule to be adsorbed. To prepare desired polymorphs these parameters are varied. However, many polymorphs are difficult to access within the experimentally available temperature-pressure ranges. In this contribution, we investigate how electric fields can be used as an additional lever to make certain structures more readily accessible. On the example of tetracyanoethylene (TCNE) on Cu(111), we analyze how electric fields change the energy landscape of interface systems. TCNE on Cu(111) can form either lying or standing polymorphs, which exhibit significantly different work functions. We combine first-principles calculations with a machine-learning based structure search algorithm and ab initio thermodynamics to demonstrate that electric fields can be exploited to shift the temperature of the phase transition between standing and lying polymorphs by up to 100 K.
Stacked organic optoelectronic devices make use of electrode materials with different work functions, leading to efficient large area light emission. In contrast, lateral electrode arrangements offer the possibility to be shaped as resonant optical antennas, radiating light from subwavelength volumes. However, tailoring electronic interface properties of laterally arranged electrodes with nanoscale gaps - to e.g. optimize charge-carrier injection - is rather challenging, yet crucial for further development of highly efficient nanolight sources. Here, we demonstrate site-selective functionalization of laterally arranged micro- and nanoelectrodes by means of different self-assembled monolayers. Upon applying an electric potential across nanoscale gaps, surface-bound molecules are removed selectively from specific electrodes by oxidative desorption. Kelvin-probe force microscopy as well as photoluminescence measurements are employed to verify the success of our approach. Moreover, we obtain asymmetric current-voltage characteristics for metal-organic devices in which just one of the electrodes is coated with 1-octadecanethiol; further demonstrating the potential to tune interface properties of nanoscale objects. Our technique paves the way for laterally arranged optoelectronic devices based on selectively engineered nanoscale interfaces and in principle enables molecular assembly with defined orientation in metallic nano-gaps.
A semitheoretical formalism based on classical electromagnetic wave theory has been developed for application to the quantitative treatment of reflection spectra from multilayered anisotropic films on both metallic and nonmetallic substrates. Both internal and external reflection experiments as well as transmission can be handled. The theory is valid for all wavelengths and is appropriate, therefore, for such experiments as x‐ray reflectivity, uv–visible spectroscopic ellipsometry, and infrared reflection spectroscopy. Further, the theory is applicable to multilayered film structures of variable number of layers, each with any degree of anisotropy up to and including full biaxial symmetry. The reflectivities (and transmissivities) are obtained at each frequency by solving the wave propagation equations using a rigorous 4×4 transfer matrix method developed by Yeh in which the optical functions of each medium are described in the form of second rank (3×3) tensors. In order to obtain optical tensors
Recently it has been proposed that alkylsiloxane self-assembled monolayers on oxidized Si(100) decompose through C–C bond cleavage [G. J. Kluth, M. M. Sung, and R. Maboudian, Langmuir 13, 3775 (1991)]. To verify this desorption picture, pentadecyltrichlorosilane precursors with four deuterated carbon atoms at the end of the molecule have been synthesized. High-resolution electron energy-loss spectroscopy shows that the monolayers are stable in vacuum up to 750 K. Above 750 K the C–D stretch disappears, while the C–H stretch remains, indicating that the end of the chain desorbs before the entire chain. From these observations it is concluded that the chains decompose primarily through C–C bond cleavage, resulting in the desorption of hydrocarbon fragments and a reduction in chain length. © 1998 American Vacuum Society.
This introduction to the physics of "Surfaces and Interfaces of Solids" emphasizes both experimental and theoretical aspects of the subject. Beside the techniques of preparing well-defined solid surfaces and interfaces basic models for the description of structural, vibronic and electronic properties of interfaces are described, as well as fundamental aspects of adsorption and layer growth. Because of its importance for modern microelectronics special emphasis is placed on the electronic properties of semiconductor interfaces and heterostructures. Experimental topics covering the basics of ultrahigh-vacuum technology, electron optics, surface spectroscopies and electrical interface characterization techniques are presented in the form of separate panels. This format allows a balanced treatment both of theory and experiment in this highly interdisciplinary field of interface physics. Graduate students and researchers who want to enter or are working in field like surface physics, materials science, semiconductor technology or microelectronics might benefit from the comprehensive treatment of the subject. The author describes, in a comprehensive way, simple models and experimental results of interface physics. Morphology and structure as well as vibronic and electronic properties of solid surfaces and interfaces are discussed. Beside the preparation of well defined surfaces a chapter treats scattering processes which have contributed significantly to the present understanding. Particular emphasis is placed on semiconductor spacecharge layers and heterostructures because of their importance for microelectronics. Experimental techniques are presented in the form of separate panels.
The interfacial properties of organic materials are of critical importance in many applications, especially the control of wettability, adhesion, tribology, and corrosion. The relationships between the microscopic structure of an organic surface and its macroscopic physical properties are, however, only poorly understood. This short review presents a model system that has the ease of preparation and the structural definition required to provide a firm understanding of interfacial phenomena. Long‐chain thiols, HS(CH2)nX, adsorb from solution onto gold and form densely packed, oriented monolayers. By varying the terminal functional group, X, of the thiol, organic surfaces can be created having a wide range of structures and properties. More complex systems can be constructed by coadsorbing two or more thiols with different terminal functional groups or with different chain lengths onto a common gold substrate. By these techniques, controlled degrees of disorder can be introduced into model surfaces. We have used these systems to explore the relationships between the microscopic structure of the monolayers on a molecular and supramolecutar scale and their macroscopic properties. Wettability is a macroscopic interfacial property that has proven of particular interest.
We have used grazing incidence x-ray diffraction to study the structure of the self-assembled monolayer of CH3(CH2)17SH adsorbed on Au(111) and Ag(111). We find that the structure of the monolayer is very different on each of these surfaces. Although on Au(111) the monolayer forms a commensurate √3×√3R30° structure, on Ag(111) the monolayer is both incommensurate and rotated, with a denser lattice spacing.
Monomers from the mercaptan and silane groups have been applied as monomolecular layers according to the Langmuir-Blodgett technique, i.e. the two-phase cell. The coatings featured a high electrochemical stability as well as a good resistance to corrosion. Detailed fundamental research has given information about the phase-boundary reactions. The tests have confirmed the concept of development of a chemically modified steel surface. Further investigations should lead to the development of corresponding coating systems and of processes for their application. This concept is interesting not only for plastic coatings but also for adhesive agents, which are used to an increasing extent in the automotive industry.
Equimolar, binary-component, self-assembled monolayers (SAMs) of-S(CHz)15CHzOH and-S(CHz)(lb+m)-CH3, with m varied systematically from-6 to +6, were prepared on evaporated gold films by self-assembly from ethanol solutions of the corresponding thiols. Single-wavelength ellipsometry, X-ray photoelectron spectroscopy, and infrared reflection spectroscopy (IRS) were used to monitor the coverage, composition, and chain conformational order, respectively, in the air1SAM structures, while liquid drop contact angle measurements with hydrogen-bonding and hydrocarbon liquids were used to probe the structural and chemical characteristics at the 1iquidSAM interface. Both the IRS C-H stretching mode characteristics and the hydrocarbon liquid contact angles show sharp changes as m increases through the region of-4 to 0, while the hydrogen-bonding liquid contact angles remain essentially constant across the range of m values. The combined data lead to a general structural model in which monolayer structures shift from randomly placed, protruding HO-terminated chain segments which screen the underlying CH3 surface at low m to contiguously placed, protruding CH3-terminated chains, organized in clusters and extending away from the underlying CHzOH surface at high m. These structures require that the monolayers approach random mixing at low m but be phase-segregated at high m.
We report the use of metastable Ar(3P0,2) atoms and a physical mask to pattern octadecylsiloxane self-assembled monolayers grown directly on silicon surfaces. The damage to the monolayer is confirmed using lateral force microscopy, changes in hydrophilicity, and x-ray photoelectron spectroscopy analysis. Metastable atom exposures sufficient to uniformly damage the monolayer should allow pattern transfer to the underlying Si(100) substrate following chemical and plasma etching. With optical manipulation of the incident metastable atoms, this technique could provide the basis for massively parallel nanoscale fabrication on silicon.
Undoped or Y 2O 3-doped ZrO 2 thin films were deposited on self-assembled monolayers (SAMs) with either sulfonate or methyl terminal functionalities on single-crystal silicon substrates. The undoped films were formed by enhanced hydrolysis of zirconium sulfate (Zr(SO 4)-4H 4O) solutions in the presence of HCl at 70°C. Typically, these films were a mixture of two phases: nanocrystalline tetragonal- (t-) ZrO 2 and an amorphous basic zirconium sulfate. However, films with little or no amorphous material could be produced. The mechanism of film formation and the growth kinetics have been explained through a coagulation model involving homogeneous nucleation, particle adhesion, and aggregation onto the substrate. Annealing of these films at 500°C led to complete crystallization to t-ZrO 2. Amorphous Y 2O 3-containing ZrO 2 films were prepared from a precursor solution containing zirconium sulfate, yttrium sulfate (Y 2(SO 4) 3•8H 2O), and urea (NH 2CONH 2) at pH 2.2-3.0 at 80°C. These films also were fully crystalline after annealing at 500°C.
Dense, homogeneous, and complete self-assembled monolayers (SAMs) with epoxy surface groups were fabricated from epoxysilanes to serve as a template for chemical anchoring of polymer layers. A combination of scanning probe microscopy, ellipsometry, XPS, X-ray reflectivity, and contact angle measurements was used to study their morphology and surface properties. Self-assembly of epoxysilane molecules resulted in the formation of homogeneous SAMs 0.85 nm thick with the surface roughness 0.22 nm. Epoxysilane SAMs were truly monomolecular films with a virtually normal molecular orientation of densely packed molecules, which were firmly attached to the substrate. The formation of stable polymer layers from carboxy-terminated polystyrenes on reactive SAM surfaces was demonstrated.
The presence of two sulfur species was detected in X-ray photoelectron spectroscopy (XPS) studies of thiol and disulfide molecules adsorbed onto gold surfaces. These species are assigned to bound thiolate (S2p3/2 binding energy of 162 eV) and unbound thiol/disulfide (S2p3/2 binding energy from 163.5 to 164 eV). These assignments are consistent with XPS data obtained from different thiols (C12, C16, C18, and C22 alkane thiols, a fluorinated thiol, and a cyclic polysiloxane thiol) and different adsorption conditions (solvent type, thiol concentration, temperature, and rinsing). In particular, the use of a poor solvent for thiol adsorption solutions (e.g., ethanol for long chain alkanethiols) and the lack of a rinsing step both resulted in unbound thiol molecules present at the surface of the bound thiolate monolayer. This has implications for recent studies asserting the presence of multiple binding sites for gold−thiolate species in organic monolayers.
Self-assembly of n-dioctadecyl sulfide (ODS) on Au(111) has been closely investigated by using X-ray photoelectron spectroscopy (XPS), in which the binding condition of sulfur on Au(111) was determined by the S(2p) XPS peak position, and the surface density and chain conformation of adsorbed molecules were determined by the relative XPS peak intensity, C(1s)/S(2p). The surface reaction of ODS on Au(111) was unstable unlike ODT SAM, and it was changed drastically by small variation of adsorption condition. When adsorption was carried out in 1 mM CH2Cl2 solution at room temperature, ODS molecules mostly formed fully adsorbed SAMs, intact without C-S cleavage. This was evaluated by the C(1s)/S(2p) intensity, which was twice as strong as ODT SAM, and by the S(2p) peak which appeared as a doublet at the position of 'unbound' sulfur [S(2p3/2) at approx. 163 eV], suggesting 'physisorption' of ODS on Au(111). On the other hand, when a different condition for SAM formation was used (e.g., high temperature, long time immersion, or CHCl3 as a solvent), the C(1s)/S(2p) intensity decreased to a value smaller than ODT SAM, and the S(2p) peak was shifted to lower binding energies, the 'bound' (162 eV) and 'free' (161 eV) sulfur positions. In these SAMs, different surface reactions including carbon-sulfur (C-S) bond cleavage seem to occur rather than nondestructive adsorption. High-resolution atomic force microscope images revealed that ODS SAM, prepared by 24-h immersion in 1 mM CH2Cl2 solution at room temperature, formed a hexagonal lattice with the lattice constant, d = 0.46 nm, which is nearly equal to the close-packed distance between alkyl chains and totally incommensurate against gold adlattice. Our data suggest a unique self-assembling process of ODS SAM, in which the chain-chain interaction is expected to be more predominant rather than the molecule-substrate interaction unlike ODT SAM.
The formation of intricate nanostructures will require the ability to maintain surface registry during several patterning
steps. A scanning probe method, dip-pen nanolithography (DPN), can be used to pattern monolayers of different organic molecules
down to a 5-nanometer separation. An “overwriting” capability of DPN allows one nanostructure to be generated and the areas
surrounding that nanostructure to be filled in with a second type of “ink.”
The crystalline structure of hydroxy‐functionalized hexanethiol self‐assembled monolayers on Au(111) were characterized using gas‐phase transport of molecular precursors and an ultrahigh vacuum scanning tunneling microscope. In the pristine state the monolayer exhibits a commensurate lattice with an oblique primitive unit‐cell of dimensions a=3, b=√13, α=a tan(2√3); a structure that is fundamentally different than the hexagonal lattice observed for methyl‐terminated monolayers. Hydration of the monolayer results in conversion to a polymorphic phase. These results demonstrate the importance of end‐group chemical bonding in the molecular packing of this widely studied class of materials.
Atomic force microscopy (AFM), contact angle, and ellipsometry measurements art: used to investigate the growth behavior of n-octadecyltrichlorosilane (OTS), n-propyltrichlorosilane (PTS), and n-triacontyltrichlorosilane (TCTS) films on hydroxylated Si(100) substrates. AFM images show that self-assembled monolayers of OTS (CH3(CH2)(17)SiCl3) grow via islands. After an initial nucleation and growth of larger primary OTS islands, smaller secondary islands grow in the areas between the primary islands until the film is complete. The shape and size of the islands and the progress in growth are not well-defined, however, and depend upon the film preparation conditions. In contrast, shorter-chain PTS molecules (CH3(CH2)(2)SiCl3) do not appear to demonstrate this island growth behavior during film formation. Longer-chain TCTS molecules (CH3(CH2)(29)SiCl3) show an island-type of growth with different island structures compared to those observed for OTS. We discuss the differences in growth behavior observed under cleanroom and normal laboratory conditions.
Monolayers of 12-(4-nitroanilino)-dodecane thiol
[O2N-C6H4-NH(CH2)12SH](NAT)
adsorbed on polycrystalline gold substrates were investigated in a
gaseous environment and in ethanol. Contributions of comparable
magnitude from the metal substrate, the adsorbate-substrate interaction,
and the p-nitro aniline moiety (pNA) require phase-sensitive and
polarization-dependent second-harmonic-generation (SHG) measurements.
The susceptibility of the pNA end group is separated by comparative
experiments with n-alkane thiols. Evaluation of the orientation of the
pNA moiety requires phase information about the SHG signal. Intensity
data alone produce ambiguous results. The tilt angle determined by SHG
is shown to be strongly dependent on the model applied. For the NAT film
in an inert gas atmosphere, the tilt angle ϑ of the pNA group
calculated from the SHG experiments is compared with the value of
53° obtained by linear techniques. Agreement is only achieved if
local-field factors are neglected and if a two-layer model is assumed to
describe the linear optical properties of the system. Immersion of the
NAT film in ethanol causes a change of the tilt angle of the pNA end
group to a more upright position of about ϑ=36°. A detailed
discussion reveals that the accuracy and reliability of SHG data to
determine the orientation of molecules depend critically on the
precision by which molecular properties are known and on the model
describing the linear optical properties.
The formation process of a self-assembled monolayer, deposited from
solution, has been observed in real time using atomic force microscopy.
The growth begins with the nucleation of submonolayer islands, which
grow and eventually coalesce. By following the time dependence of the
island nucleation and the growth kinetics of individual islands, we find
that the process can be quantitatively explained by a kinetic theory of
2D cluster growth typically used to describe vapor phase molecular beam
epitaxy. The same theory correctly predicts the maximum island density
prior to island coalescence.
In situ atomic force microscope observations of the formation of
octadecylphosphonic acid monolayers, deposited from solution onto mica,
indicate that growth proceeds via the nucleation, growth, and
coalescence of densely packed submonolayer islands of adsorbate
molecules. Three regimes are observed: (1) an initial growth regime
where nucleation of new islands is significant, (2) an aggregation
regime where nucleation essentially stops and existing islands grow, and
(3) a coalescence regime where individual islands merge, resulting in
fewer islands. In analogy with vapor phase thin-film deposition (such as
molecular-beam epitaxy) the island size distribution in the aggregation
regime is predicted to show dynamic scaling behavior, indicating that at
a given time, only one length scale is present. We explicitly verify
this dynamic scaling assumption, showing that the island size
distributions, over a range of surface coverage from 0.06-0.17, can be
collapsed into a single dimensionless distribution function by the
theoretically predicted scaling relationships. The shape of this
distribution function implies that Ostwald ripening is not a significant
factor and that the critical nucleus is <=2 molecules.
For a decade now the subject of the nature of the two-dimensional melting transition has remained controversial. An elegant theory based on the unbinding of pairs of crystal defects suggested that two-dimensional solids might melt by a transition sequence involving two continuous transitions separated by a novel, nearest-neighbor-bond-orientationally ordered fluid—the hexatic phase. Competing theories predict that the transition is of the usual first-order type observed in three-dimensional systems. This paper is a critical review of the current status of research into the problem of two-dimensional melting, with an emphasis on computer simulations. An attempt is made to point out unresolved issues pertaining to this fascinating and still open question.
Alkylsilane lubricant films under pressure were used to investigate how friction affects molecular structure. Results showed that molecular events happening at specific threshold loads may cause the energy losses observed when alkylsilane self-assembled monolayers are in friction with sharp atomic force microscope tips. The self-assembled islands of alkylsilanes on mica experienced a stepwise decrease in height. It is suggested that the stepwise height decrease is a function of imaging pressure.
An X-ray study of a crystalline organic–organic heterostructure, consisting of a film of PTCDA (3,4,9,10-perylenetetracarboxylic dianhydride) grown on a self-assembled monolayer (SAM) of decanethiol on Au(1 1 1), is presented. The sandwich structure acts as a “molecular interferometer”, where the interference between the Laue function of the PTCDA film and the crystal truncation rod (CTR) of the substrate is governed by the SAM spacer thickness, d0. A pronounced destructive interference feature is observed, which allows the determination of d0 with great sensitivity.
Surface structures of submonolayer alkanethiol CH3(CH2)n−1SH (n=6,12) adsorbed on Cu(111) at room temperature (0.3 ML) were investigated by means of S K-edge surface extended X-ray absorption fine structure (EXAFS) and S, C K-edge near-edge X-ray absorption fine structure (NEXAFS) spectroscopies. Polarization dependence of the C K-edge NEXAFS spectra revealed that alkanethiol molecules adsorb with the tilt angle of 12 ± 10° from the surface normal. The S K-edge surface EXAFS determined that the SCu distance is and that the S atoms locate at a deep three-fold hollow site with significant lateral outward movements of the nearest neighbor Cu atoms.
The structure of alkylsiloxane self-assembled monolayers formed on HF-treated Si3N4 has been studied using x-ray photoelectron spectroscopy, high-resolution electron energy-loss spectroscopy, and contact angle analysis. It is shown that the monolayers are similar in quality to those formed on oxidized silicon, despite the fact that upon etching in HF, the Si3N4 surface contains only 0.2 ML of oxygen. In contrast, on NH4F-treated Si(100) surfaces with similar quantities of oxygen, high-quality monolayers cannot be formed. We argue that these results point to the importance of a water layer in monolayer formation. © 1999 American Vacuum Society.
The adsorption and desorption of n-alkanethiol monolayers on Au(111) have been studied under ultrahigh-vacuum condition by the use of scanning tunneling microscopy (STM), thermal desorption spectroscopy (TDS), and Auger electron spectroscopy (AES). Molecularly resolved STM observations for the alkanethiol monolayers have revealed that at least four different phases evolve during growth, which results in a multistep growth of the monolayer. The desorption species drastically changes at a critical coverage, which is accompanied by a structure change from a low-density flat-lying phase to a denser standing-up phase: While the latter phase bimolecularly desorbs as disulfides, the former phase unimolecularly desorbs as thiolate radicals. The coverage-dependent change of the desorption mode is explained in terms of the difference in the molecule-substrate bonding.
I CANNOT but think that my friend Mr. Bottomley is a little hard on Prof. Van der Waals. I am not aware that there is any dispute as to the fact that the methods he employed are open to criticism, and that his formula is only approximately true. In spite of its defects the treatise was regarded by Maxwell at the time of its publication as of very great interest. If, however, Van der Waals is accused of not showing a ``proper appreciation of the work of Andrews,'' the following facts should be considered before judgment is passed:-
We present the results of a combined He atom and x-ray diffraction study of CH3(CH2)n−1SH monolayers self assembled on Au(111) surfaces. By combining these two complementary probes, we have characterized both the surface and the interior structure of the monolayers. In both cases, we find the same structure containing four molecules per unit mesh. However, we demonstrate that there are significant differences in both the diffraction linewidths and the dependence of the linewidth upon chain length for these two techniques.
A number of compounds in which a hydrocarbon chain is attached to a single sulphur- or selenium-containing group have been prepared, for test as promoters of dropwise condensation of steam.
High-resolution electron-energy-loss spectra of octadecanethiol self-assembled monolayers (SAM's) on Au thin films have been obtained after annealing the sample to various temperatures. Annealing to 375 K results in the appearance of the S-S stretch at 530 ${\mathrm{cm}}^{\mathrm{$-${}}1}$, a direct observation of sulfur dimers for alkanethiol SAM's adsorbed on Au. The appearance of dimers following annealing is explained by the presence of an activation barrier to the formation of gauche defects at the S-C bond.
By the enclosed from an old friend, a worthy clergyman at Carlisle, whose great learning and extensive knowledge in most sciences would have more distinguished him, had he been placed in a more conspicuous point of view, you will find that he had heard of your experiment on Derwent Lake, and has thrown together what he could collect on that subject; to which I have subjoined one experiment from the relation of another Gentleman.
The epitaxial growth of ultrathin organic films (alkane thiolates) on single crystalline metal surfaces is investigated via low energy electron diffraction (LEED) with special emphasis on nonane [CH3(CH2)8SH] molecules, vacuum deposited on Au(111) surfaces. As a function of increasing coverage, the unit cells of nonane thiol can be described by c(21×√3) and p(5×√3). In between and without further change in coverage, the p(5×√3) is changed into a c(10×2√3)R30° structure. The structural changes are interpreted as being due to a decrease of tilt angle of the molecules with respect to the surface normal, followed by a rotation of the molecules with respect to the C–C–C plane of the carbon chains.
The formation of self-assembled monolayers of octadecylsiloxane adsorbed from dilute solutions of octadecyltrichlorosilane in toluene onto freshly cleaved mica surfaces was investigated using atomic force microscopy (AFM) in tapping mode as a well-suited tool to obtain local information on the adsorption process. Three different measurement methods have been used: ex situ measurements and in situ measurements under stopped flow/deposition as well as continuous flow/deposition conditions. Although valuable information on the growth process can be obtained under stable and reproducible conditions with all methods addressed, in situ measurements bear a number of significant advantages for the investigation of such dynamic processes.
Zusammenfassung Untersuchungen des Wrmebergangs von kondensierendem Dampf an ebene Kupferplatten, deren andere Seite durch einen senkrecht
darauf gerichteten freien Wasserstrahl gekhlt wird. Je nach Beschaffenheit der Oberflche Kondensation in Einzeltropfen oder
als zusammenhngender Wasserfilm.—Messungen der Wrmebergangszahl auf beiden Seiten der Platte. Untersuchung der Abhngigkeit
der Wrmebergangszahl auf der Wasserseite von der Geschwindigkeit des Khlwasserstrahls und dem Abstand von der Strahlmitte.—Messung
der Dicke der Khlwasserhaut.—Vergleich der Versuchsergebnisse mit der Nusseltschen Theorie.
Results are presented of Monte Carlo calculations for self-assembled monolayer systems consisting of binary mixtures of long-chain alkyl thiols of different lengths adsorbed on a gold surface. A sampling scheme is used in which largescale conformational changes are made to a trial molecule in a single move, supplemented by attempted interconversions of long and short chains. A marked trend towards segregation of the two species is observed, and the outer regions of the monolayers are found to be conformationally more disordered than the inner parts; both effects are most pronounced when the concentration of long chains is low.
The origin of frictional forces in self-assembled monolayers (SAMs) was investigated through systematic correlation of the frictional properties with the chemical structure/composition of the films. Atomic force microscopy was used to probe the frictional properties of the SAMs formed by the adsorption of methyl-, isopropyl-, and trifluoromethyl-terminated alkanethiols on Au(111) surfaces. The frictional properties of mixed monolayers composed of varying concentrations of the methyl- and trifluoromethyl-terminated thiols were also studied. Polarization modulation infrared reflection adsorption spectroscopy was used to measure the vibrational spectra of each of these monolayers and in turn to determine that each was characterized by a well-packed backbone structure. For these films, which differed only in the nature of the outermost chemical functionality, a substantial enhancement in the frictional response was observed for films with isopropyl- and trifluoromethyl-terminal groups and for mixed monolayers containing small concentrations of the trifluoromethyl-terminated component. These results strongly support the model that the difference in friction in such systems arises predominantly from the difference in the size of the terminal groups. Larger terminal groups in films of the same lattice spacing give rise to increased steric interactions that provide pathways for energy dissipation during sliding.
We report on the first electron spin resonance (ESR) investigation of the molecular rotational motion of self-assembled monolayers. We present results on fatty acid films grown on a thin Al2O3 film. The spectra reveal a distinct temperature and coverage dependence. The changes in the line shape with variation of the spin labels location along the alkyl chain allow insight into the internal dynamics of self-assembled films.
Docosaneselenol in solution spontaneously forms an ordered (self-assembled (SA)) monolayer on the gold(111) surface. The contact wetting angle of hexadecane (53-55-degrees) on the SA monolayer coated Au surface is consistent with a surface exposing methyl groups which are at the end of the C22 chains. The in-plane structure of this SA monolayer was probed by surface X-ray diffraction. The SA monolayer forms an incommensurate structure with an oblique unit cell with a = 5.204 angstrom, b = 4.897 angstrom, and gamma = 120-degrees. The (a over arrow pointing right + b over arrow pointing right) axis is along the Au [202BAR] direction. The correlation length obtained from the radial scan was approximately 60 angstrom. The tilt angle measured from the surface normal was 15-degrees with the chains tilted along the Au [202BAR] direction (i.e. R30-degrees). The oblique structure of the unit cell represents a distorted hexagonal close packed lattice.
Time-resolved atomic force microscopy and contact angle measurements reveal that the self-assembly of octadecyltrichlorosilane-based monolayers on the oxidized Si(100) surface can be initiated by three distinct mechanisms: island growth at low temperatures (T < 16 degrees C), homogeneous growth at high temperatures (T greater than or equal to 40 degrees C), and a mixed regime at intermediate temperatures. These results support the notion of a transient Langmuir film at the substrate-solution interface: the observed growth patterns are consistent with nucleation of liquid-condensed domains coexisting with a two-dimensional gas at low temperature, or with a liquid-expanded phase at intermediate temperatures.
Near-edge X-ray absorption fine structure (NEXAFS) and temperature-programmed desorption (TPD) studies have been performed to establish the relationship between adsorbate structure and binding energy in a monolayer of hydrocarbons on a Cu(100) surface. Fourteen different saturated and monounsaturated hydrocarbons were studied. The activation energy for desorption of these compounds has been found to be dependent on the following factors: (1) the length of the saturated hydrocarbon linear chain; (2) the presence and location of a double bond; (3) the cyclic versus acyclic nature of the hydrocarbons; and (4) the accessibility of the CHn groups for creating van der Waals interactions with the surface. Similar to previous observations on other surfaces, our results show that an increase of the linear hydrocarbon chain length by one methylene group increases the binding energy of a hydrocarbon by 1.5 kcal/mol. Our results also indicate that the presence of a double bond in a position where overlap between π-orbitals of a hydrocarbon and d-orbitals of the metal is significant (double bond parallel to the Cu(100) surface) increase the binding energy of an olefin molecule by 0.75 kcal/mol with respect to that of the corresponding saturated hydrocarbon.
Self-assembled monolayers of decanethiol on Au(111) have been investigated using high resolution scanning tunneling microscopy (STM). After the annealing of the as-adsorbed densely packed c(23×43)R30° structure and before the formation of a previously described low density (11.5×3) stripe phase three different phases with intermediate molecular surface density have been observed. We present highly resolved and distortion-corrected STM images for both the (7.5×3) stripe phase and the meshlike structure. We describe a mixed (7.5/11.5×3) stripe phase that shows a switching of the stripe arrangement.
A theory of sticking, based on the kinetic lattice gas that accounts for intrinsic and extrinsic precursors and incorporates the effects of lateral interactions, is used to develop a model for the sticking of rare gases on metals and to derive analytically the model of Zeppenfeld et al. [Surf. Sci. 318 (1994) L1187] that explains their data as due to the formation and coalescence of condensed islands in the adsorbate.