Self-assembled Monolayer Packing Research Articles

Dynamically blocking leakage current in molecular tunneling junctions.

Molecular tunneling junctions based on self-assembled monolayers (SAMs) have demonstrated rectifying characteristics at the nanoscale that can hardly be achieved using traditional approaches. However, defects in SAMs result in high leakage when applying bias. The poor performance of molecular diodes compared to silicon or thin-film devices limits their further development. In this study, we show that incorporating "mixed backbones" with flexible-rigid structures into molecular junctions can dynamically block tunneling currents, which is difficult to realize using non-molecular technology. Our idea is achieved by the interaction between interfacial dipole moments and electric field, triggering structured packing in SAMs. Efficient blocking of leakage by more than an order of magnitude leads to a significant enhancement of the rectification ratio to the initial value. The rearrangement of supramolecular structures has also been verified through electrochemistry and electroluminescence measurements. Moreover, the enhanced rectification is extended to various challenging environments, including endurance measurements, bending of electrodes, and rough electrodes, thus demonstrating the feasibility of the dynamic behavior of molecules for practical electronic applications.

Supramolecular tunnelling junctions with robust high rectification based on assembly effects.

The performance of large-area molecular diodes can in rare cases approach the lower limit of commercial semiconductor devices but predictive structure-property design remains difficult as the rectification ratio (R) achieved by self-assembled monolayer (SAM) based diodes depends on several intertwined parameters. This paper describes a systematic approach to achieve high rectification in bisferrocenyl-based molecular diodes, HSCnFc-CC-Fc (n = 9-15) immobilised on metal surfaces (Ag, Au and Pt). Experiments supported by molecular dynamics simulations show that the molecular length and bottom electrode influence the SAM packing, which affects the breakdown voltage (VBD), the associated maximum R (Rmax), and the bias at which the Rmax is achieved (Vsat,R). From the electrical characterisation of the most stable Pt-SCnFc-CC-Fc//GaOx/EGaIn junctions, we found that VBD, Vsat,R, and Rmax all scale linearly with the spacer length of Cn, and that Rmax for all the SAMs consistently exceeds the "Landauer limit" of 103. Our data shows that the robust switching of M-SCnFc-CC-Fc//GaOx/EGaIn junctions is the result of the combined optimisation of parameters involving the molecular structure, the type of metal substrate, and the applied operating conditions (bias window), to create stable and high-performance junctions.

Parity Effects in the Physicochemical Properties of Self-Assembled Monolayers

Large-area metal/molecule/metal junctions based on self-assembled monolayers (SAMs) of organothiolates are of interest for investigations of charge transport across ultrathin organic films and the fabrication of molecular electronic devices. Several recent reports demonstrate an odd-even dependency in the rate of charge transport through SAMs of the insulating n-alkanethiolates (CH3C n S)1,2 or the redox-active ferrocenylalkanethiolates (FcC n S)3. This parity effect has been attributed to differences in the orientation of the terminal functional group and alkyl chain packing in SAMs consisting of an odd versus even number of methylene repeat units.We have sought experimental evidence for odd-even distinctions in the molecular organization of these SAMs using infrared reflection-adsorption spectroscopy (IRRAS), electrochemistry, and contact angle goniometry. The findings of the investigations will be presented in this talk and the molecular origin of the parity effect of SAM macroscopic properties will be discussed.

Stereostructural Effects in the Electrochemical Properties of Self-Assembled Monolayers

Large-area metal/molecule/metal junctions based on self-assembled monolayers (SAMs) of organothiolates are of interest for the investigation of charge transport across ultrathin organic films and the fabrication of molecular electronic devices such as diodes and switches. Several reports demonstrate an odd-even dependency in the rate of charge transport across SAMs of the redox-active ferrocenylalkanethiolates Fc(CH2) n S1, 2 and insulating n-alkanethiolates CH3(CH2) n S3-6. This parity effect is attributed to small changes in the orientation of the terminal functional group (ferrocene or methyl) and in the alkyl chain packing in SAMs consisting of an odd versus even number of methylene repeat units (n odd or n even). These odd-even differences ultimately impact device performance. For instance, junctions comprising gold-supported Fc(CH2) n S SAMs (Figure 1) of n even present lower leakage currents and rectify current, while those of n odd do not.1, 2 We have sought experimental evidence for odd-even distinctions in the molecular organization of these SAMs using electrochemistry,7 infrared reflection-absorption spectroscopy (IRRAS), contact angle goniometry, and surface stress measurements8. The findings of these investigations will be presented in this talk.

Enhanced Performance of Self‐Assembled Monolayer Field‐Effect Transistors with Top‐Contact Geometry through Molecular Tailoring, Heated Assembly, and Thermal Annealing

Low‐voltage self‐assembled monolayer field‐effect transistors (SAMFETs) that operate under an applied bias of less than −3 V and a high hole mobility of 10−2 cm2 V−1 s−1 are reported. A self‐assembled monolayer (SAM) with a quaterthiophene semiconducting core and a phosphonic acid binding group is used to fabricate SAMFETs on both high‐voltage (AlOx/300 nm SiO2) and low‐voltage (HfO2) dielectric platforms. High performance is achieved through enhanced SAM packing density via a heated assembly process and through improved electrical contact between SAM semiconductor and metal electrodes. Enhanced electrical contact is obtained by utilizing a functional methylthio head group combined with thermal annealing post gold source/drain electrode deposition to facilitate the interaction between SAM and electrode.

Influence of self-assembled monolayer binding group on graphene transistors

Graphene transistors on self-assembled monolayer (SAM) modified dielectric substrates were fabricated and characterized in order to determine the influence SAM binding group has on device properties. It was found that silane based alkyl SAMs had little to no influence in doping graphene transistors, while phosphonic acid based ones caused n-type doping of graphene transistors with a charge neutrality point shift of over 10 V. It was also discovered that alkyl SAM packing density influenced the doping magnitude. Due to substrate surface charge trap quenching, these SAMs independent of binding group enhanced charge mobility of graphene transistors compared to ones on bare oxide substrates.

Salt-mediated self-assembly of thioctic acid on gold nanoparticles.

Self-assembled monolayer (SAM) modification is a widely used method to improve the functionality and stability of bulk and nanoscale materials. For instance, the chemical compatibility and utility of solution-phase nanoparticles are often improved using covalently bound SAMs. Herein, solution-phase gold nanoparticles are modified with thioctic acid SAMs in the presence and absence of salt. Molecular packing density on the nanoparticle surfaces is estimated using X-ray photoelectron spectroscopy and increases by ∼20% when molecular self-assembly occurs in the presence versus the absence of salt. We hypothesize that as the ionic strength of the solution increases, pinhole and collapsed-site defects in the SAM are more easily accessible as the electrostatic interaction energy between adjacent molecules decreases, thereby facilitating the subsequent assembly of additional thioctic acid molecules. Significantly, increased SAM packing densities increase the stability of functionalized gold nanoparticles by a factor of 2 relative to nanoparticles functionalized in the absence of salt. These results are expected to improve the reproducible functionalization of solution-phase nanomaterials for various applications.

Surface characterization and platelet compatibility evaluation of the binary mixed self-assembled monolayers

This report describes a technique that used mixed self-assembled monolayer (SAM) as a model surface to evaluate the effect of steric hindrance on the SAM packing quality and its platelet compatibility. Two series of binary mixed SAMs were formed by mixing the bulky terminated alkanethiol (HS(CH 2) 10PO 3H 2) with a smaller terminated one (HS(CH 2) 9CH 3 and HS(CH 2) 11OH) respectively. Surface characterization results showed the hydrophilicity on these two series of mixed SAMs changed with the solution mole fraction of PO 3H 2 terminated thiol, χ PO 3H 2,soln , and reached to a nearly constant value as χ PO 3H 2,soln was 0.6 for PO 3H 2 + CH 3 SAM and 0.4 for PO 3H 2 + OH SAM. This finding should be due to the gradual saturation of surface PO 3H 2 functionality on these mixed SAMs. The XPS analysis indicated the addition of the CH 3 and OH terminated thiol could reduce the steric hindrance effect of PO 3H 2 functionality on monolayer formation and, henceforth, improve the SAM packing quality. In vitro platelet adhesion assay revealed the platelet compatibility on the PO 3H 2 + OH SAMs was better than that on the PO 3H 2 + CH 3 and the pure PO 3H 2 ones. Moreover, the PO 3H 2 + OH SAM with a low χ PO 3H 2,soln value exhibited the least platelet activating property of these two mixed SAM systems. These findings suggested that material's surface wettability and surface charge density should act collectively in affecting its platelet compatibility.

In-situ examination of the selective etching of an alkanethiol monolayer covered Au{111} surface

An examination of the selective etching mechanism of a 1-alkanethiol self-assembled monolayer (SAM) covered Au{111} surface using in-situ atomic force microscopy (AFM) and molecular resolution scanning tunnelling microscopy (STM) is presented. The monolayer self-assembles on a smooth Au{111} surface and typically contains nanoscale non-uniformities such as pinholes, domain boundaries and monatomic depressions. During etching in a ferri/ferrocyanide water-based etchant, selective and preferential etching occurs at SAM covered Au(111) terrace and step edges where a lower SAM packing density is observed, resulting in triangular islands being relieved. The triangular islands are commensurate with the Au(111) lattice with their long edges parallel to its [11¯0] direction. Thus, SAM etching is selective and preferential attack is localized to defects and step edges at sites of high molecular density contrast.

Dynamic and Collective Electrochemical Responses of Tetrathiafulvalene Derivative Self-Assembled Monolayers

Electroactive tetrathiafulvalene (TTF)-containing alkanethiol self-assembled monolayers (SAMs) were designed and synthesized to elucidate the relationship between electrochemical responses and film structures. Two TTF derivative molecules having one alkanethiol chain (1) and two alkanethiol chains (2) were utilized to modulate the molecular packing arrangements in the SAMs, and the formation and structure of the SAMs were characterized by surface plasmon resonance spectroscopy (SPR). SPR measurements in various contacting media demonstrated loose packing of SAM 1 and close packing of SAM 2 due to the different space fillings of the molecules. Two successive one-electron redox waves were observed for both SAMs by cyclic voltammetry. The peak widths of the redox waves were strongly dependent on the oxidation states of the TTF moieties, the packing arrangement of the SAMs, and the contacting medium. We found that TTF-based SAMs exhibited collective electrochemical responses induced by dynamic structural changes, depending on the degree of freedom for the component molecules in the SAMs. These results imply that the molecular design, taking into account the electrochemical responses, extends the available range of molecular-based functionalities in TTF-based SAMs.

Preconditioning Gold Substrates Influences Organothiol Self-assembled Monolayer (SAM) Formation

Self-assembled monolayer (SAM) formation of alkanethiols with ionic, hydrophilic terminal functionalities onto various O2 plasma/ethanol pretreated gold substrates was characterized to explore the effect of gold surface oxide on the SAM packing quality. Oxygen adsorption induced by the Au2O3 surface residuals are observed on the plasma-oxidized and O2 plasma/ethanol-rinsed pretreated Au surfaces while no obvious adsorbed oxygen is found on freshly coated and O2 plasma/ethanol sonication pretreated Au substrates. A model for the formation of hydrophilic terminated SAMs, –OH, –COOH, and –PO3H2 is proposed. According to this model, the ionic and/or other binding interactions between the surface Au2O3 and the alkanethiol hydrophilic terminal end as well as the interactions between the terminal SAM functionalities could cause the packing disorder found on these three SAMs formed on Au substrates containing Au2O3 surface species.

Molecular packing changes of alkanethiols monolayers on Au(111) under applied pressure

A study of the changes of molecular packing in self-assembled monolayers of alkylthiols on Au(111) induced by external pressure is presented. Atomic force microscopy (AFM) is used to apply pressure and to measure the height of islands of alkanethiols partially covering the gold surface. The islands are made of ordered straight chain alkylthiol molecules tilted from the surface normal. Their height was found to decrease in a stepwise manner as a function of the load applied by the tip. Simultaneous stepwise increases in friction force were observed. A simple geometrical model involving the interlocking of alkyl chains at specific molecular tilt angles can explain the observations. According to the model, tilts in both the nearest neighbor and the next-nearest neighbor directions are necessary.

Molecular packing in self-assembled monolayers of normal alkane on Au(111) surfaces

Self-assembled monolayer of normal hexatriacontane ( n-C 36H 74) is formed at the interface between a solution of n-C 36H 74 in dodecane and a reconstructed Au(111) surface. Two kinds of lamellae have been observed, in which the long axis of n-C 36H 74 molecule is either perpendicular to or tilted 63.5° to the trough of the lamellae. The packing structures in both kinds of the lamellae are different. Theoretical calculations based on molecular mechanics have been performed to elucidate the ways of the molecular packing in the self-assembled monolayers on gold.