Self-assembled Monolayer Layer Research Articles

Electron transfer behavior at polyoxometalate-adsorbed alkanethiol self-assembled monolayers

The interaction between polyoxometalate (POM) anions, SiMo 12O 40 4−, and a self-assembled monolayer (SAM) of dodecanethiol (DT) on Au surfaces was investigated using electrochemical methods, X-ray photoelectron spectroscopy, and scanning probe microscopy. The SiMo 12O 40 4− ions adsorb on the SAM of DT on Au to form a composite organic–inorganic hybrid layer. The adsorbed SiMo 12O 40 4− ion on the SAM layer shows its characteristic redox waves with an electron transfer rate slower than that on a bare Au electrode. The electron transfer behavior at DT−SAM could be regulated by the adsorption of SiMo 12O 40 4− depending on the charge of the investigated electroactive species: a significant increase toward a positively charged Ru(NH 3) 6 3+ ion, a moderate increase toward a neutral 1,1′-ferrocenedimethanol molecule and a slight decrease toward a negatively charged Fe(CN) 6 3− ion. The effect of the chain length of alkanethiols on the adsorption of SiMo 12O 40 4− ion was also investigated: as the chain length decreases, the amount of the adsorbed POM increases and the electron transfer rate through the composite layers increases. The nature of SiMo 12O 40 4− ions adsorbed on the SAMs of alkanethiols on Au is discussed in detail.

Electrochemical Detection of Biomolecule with Mixed Self-Assembled Monolayers of Ferrocene-Undecanethiol

Thiol self-assembled monolayers (SAMs) have been widely used as a modification method to immobilize biomolecules to gold surfaces. However, the additional layers of SAM and biomolecules make electron transfer difficult, leading to a large overpotential in electrical signal. Electron transfer mediation is the most popular solution to overcome the problem of the overpotential for an electrochemical biosensor. We introduced mixed SAMs of mercapto-dodecanoic acid (MDA) and ferrocene-undecanthiol. Ferrocene-undecanthiol acts as an electron transfer mediator and MDA is used for immobilization of biomolecule. The electron transferability of mixed SAMs is affected by pH, kinds of electrolytes, and the composition of the thiol molecule. We optimized the carboxyl acid and ferrocene molecule ratio which is a crucial factor in the performance of mixed SAMs and electrochemically detected the avidin. The detection limit was 2.0 microg/mL of avidin concentration.

Low temperature Cu [sbnd]Cu thermo-compression bonding with temporary passivation of self-assembled monolayer and its bond strength enhancement

Self-assembled monolayer (SAM) of alkane-thiol is formed on copper (Cu) thin layer coated on silicon (Si) wafer with the aim to protect the surface against excessive oxidation during storage in the room ambient. After 3 days of storage, the temporary SAM layer is desorbed with in situ anneal in inert ambient to uncover the clean Cu surface. A pair of wafers is bonded at 250 °C. Clear evidences of in-plane and out-of-plane Cu grain growth are observed resulting in a wiggling bonding interface. This gives rise to enhancement in shear strength in the bonded Cu Cu layer.

High‐Resolution Surface Chemical Analysis of a Trifunctional Pattern Made by Sequential Colloidal Shadowing

We present a new method for creating surface chemical patterns where three chemistries can be periodically arranged at alternate positions on a single substrate without the use of top-down approaches. High-resolution chemical imaging by time-of-flight secondary ion mass spectrometry (ToF-SIMS), with nanometer spatial resolution, is used to prove the success of the patterning and subsequent chemical modification steps. We use a combination of colloidal self-assembly, plasma etching, self-assembled monolayers (SAMs) and physical vapour deposition (PVD). The method utilizes a double colloid assembly process in which a first layer of close-packed colloids is created, followed by plasma etching, coating with gold and deposition of a first SAM layer. A second particle layer is deposited on top of the first layer masking the interstitial spaces containing the first SAM. A second gold layer is deposited followed by a second SAM. After particle removal the surface consists of the pattern containing two different SAMs and a SiO(2) layer that can be readily functionalized with silanes. The possibility in the replacement of the two different thiols is investigated by X-ray photoelectron spectroscopy (XPS) and it was found that no replacement is taking place. ToF-SIMS imaging is used to show the periodicity of the chemical patterns by tracking unique fragment ions from the different surface regions. The patterning method is adaptable to create smaller or larger chemical patterns by appropriate choice of particle sizes. The patterns are useful for immobilizing biomolecules for cell studies or as multiplexed biosensors.

High-Performance n-Channel Organic Thin-Film Transistor for CMOS Circuits Using Electron-Donating Self-Assembled Layer

We introduce an electron-donating self-assembled monolayer (SAM) to improve the performance of solution-processed n-channel organic thin-film transistor (OTFT) using an organic semiconductor (OS) of an N, N <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">'</sup> bis-(octyl-)-dicyanoperylene-3,4:9,10-bis(dicarboximide) (PDI-8CN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ). The OTFTs without SAM, with electron-withdrawing, and electron-donating layers, exhibited field-effect mobility of 0.02, 0.01, and 0.33 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /(V·s), respectively. The electron-donating thiophenol layer on source/drain electrodes shows a small injection barrier of 0.05 eV to the n-type OS and thus, exhibited field-effect mobility of 0.33 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /(V·s) and threshold voltage of -1.1 V.

How Penetrable Are Thioalkyl Self-Assembled Monolayers?

The depth of penetration of photolytically generated, gas-phase O(3P) atoms into thioalkyl self-assembled monolayers (SAMs) has been investigated. Custom-synthesized, site-selectively deuterated SAMs were prepared on Au substrates and characterized by scanning tunneling microscopy (STM). Relative yields of gas-phase OD were detected by laser-induced fluorescence (LIF). Reaction was suppressed at the terminal CD3 by the higher abstraction barriers for primary D atoms, yielding only 16 ± 3% of the total OD. The C2 (first secondary) site is the individually most reactive (42 ± 5%). The remaining significant contribution (42 ± 4%) from positions as deep as C3−C6 is a considerable surprise when compared with previous related experiments using higher-energy O+ ion projectiles and detecting OH− products. The apparent greater penetrability of the SAM layer found here may have prior theoretical support. Furthermore, we show that NO2 damages the surfaces but that C12 SAMs are considerably more resistant than C6 SAMs.

Electrical Properties of Organic Light-Emitting Diodes with an Insertion of CMTS Self-Assembled Monolayers

Electrical properties of organic light-emitting diodes were studied in a device with an insertion of chloromethyl-trichlorosilane self-assembled monolayer (SAM) between the ITO anode and organic layer. Self-assembled monolayer is generally used for a surface reformation of ITO substrate to enhance the charge injection at the anode/organic interface. For a formation of SAM onto the ITO anode, the ITO substrate was immersed in a 20 mM solution of SAM material. SAM-treatment time on the ITO substrate was varied to be 0, 10, 15, 20, and 25 minutes. And then, the device was made in a structure of ITO(170 nm)/SAM/TPD(50 nm)/Alq3(70 nm)/LiF(0.5 nm)/Al(100 nm). A device with properly treated self-assembled monolayer at the anode/organic interface gives an improvement in turn-on voltage, luminance, and efficiency compared to the one without SAM layer. A current efficiency and an external quantum efficiency of the device with 15 minutes treated SAM layer were increased by 2.6 times compared to the ones without the SAM layer. Fowler-Nordheim tunneling conduction mechanism was applied to the current density-voltage characteristics to obtain a hole injection barrier height. It was found that there is a reduction in barrier height by 25% for the device with 15 minutes SAM-treated compared to the one without SAM-treated.

Interface dynamics and mechanisms of nanoindented alkanethiol self-assembled monolayers using molecular simulations

The interface and nanoindentation mechanisms of alkanethiol self-assembled monolayers (SAMs) chemisorbed on a gold surface are investigated using molecular dynamics simulation. The mechanisms include the nanoindentation depths, the workpiece temperatures, the numbers of SAM layers, the length of united-atoms per chain, and the shapes of the indenters. The simulation results show that the disorder and the plastic mobility of SAM chains increased with increasing indentation depth. The relaxation force and the plastic energy almost linearly increased with increasing indentation depth. The disorder region after indentation at high temperature is larger than that at low temperature. The adhesive force shows a dependence on temperature during indentation. The potential energy decreases with increasing number of SAM layers. The structural morphologies of the SAMs were not affected at the third layers for SAM film with four layers. The maximum load quickly decreases for film with two SAM layers. The structures of the SAM can be easily tilted and bent when the united-atoms per chain length is long. The SAM atoms become more disorderly and the elastic recovery is smaller when the SAM length of the united-atoms per chain is long after indentation.

Surface state control of III–V semiconductors using molecular modification

Abstract An attempt to control surface electronics of III–V semiconductor using wet chemical processes has been performed. Here, we report results on the use of self-assembled monolayers (SAMs) of organic molecules on (0 0 1) GaAs surface. Octadecanethiol (ODT) and benzenethiol (BT) have been the choice in the present study. GaAs wafers were modified by thiol molecules on the flat surface after the native oxide layers are removed by chemical etching under optimized conditions. The change in the electronic properties was measured in terms of transport properties via the SAM layer by conductive probe atomic force microscopy. The current–voltage characteristics thus obtained show that ODT functions as a tunnel barrier while BT is conductive due to the presence of π -electrons. As a result, we can control the electronic states of GaAs–molecule interface for realizing novel device structures by the selection of functional molecules.

Copper−Metal Deposition on Self Assembled Monolayer for Making Top Contacts in Molecular Electronic Devices

Molecular electronics is an attractive option for low-cost devices because it involves highly uniform self-assembly of molecules with a variety of possible functional groups. However, the potential of molecular electronics can only be turned into practical applications if reliable contacts can be established without damaging the organic layer or contaminating its interfaces. Here, a method is described to prepare tightly packed carboxyl-terminated alkyl self-assembled monolayers (SAMs) that are covalently attached to silicon surfaces and to deposit thin metallic copper top contact electrodes without damage to this layer. This method is based on a two-step procedure for SAM preparation and the implementation of atomic layer deposition (ALD) using copper di-sec-butylacetamidinate [Cu(sBu-amd)](2). In situ and ex situ infrared spectroscopy (IRS), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and electrical measurements are used to characterize the chemical modification of the Si/SAM interface, the perturbation of the SAM layer itself, and the metal homogeneity and interaction with the SAM headgroups. This work shows that (i) carboxyl-terminated alkyl monolayers can be prepared with the same high density and quality as those achieved for less versatile methyl-terminated alkyl monolayers, as evidenced by electrical properties that are not dominated by interface defects; (ii) Cu is deposited with ALD, forming a bidentate complexation between the Cu and the COOH groups during the first half cycle of the ALD reaction; and (iii) the Si/SAM interface remains chemically intact after metal deposition. The nondamaging thin Cu film deposited by ALD protects the SAM layer, making it possible to deposit a thicker metal top contact leading ultimately to a controlled preparation of molecular electronic devices.

Cu–Cu diffusion bonding enhancement at low temperature by surface passivation using self-assembled monolayer of alkane-thiol

Self-assembled monolayer (SAM) of 1-hexanethiol is applied on copper (Cu) surface to retard surface oxidation during exposure in the ambient. This SAM layer can be desorbed effectively with an annealing step in inert N2 ambient to provide a clean Cu surface. Using this passivation method with SAM, wafers covered with thin Cu layer are passivated, stored, desorbed, and bonded at 250 °C. The bonded Cu layer presents clear evidence of substantial interdiffusion and grain growth despite prolonged exposure in the ambient. This method of passivation is proven to be effective and can be further optimized to enable high quality Cu–Cu direct bonding at low temperature for application in three-dimensional integration.

Area-Selective Atomic Layer Deposition Using Self-Assembled Monolayer and Scanning Probe Lithography

We investigated area-selective atomic layer deposition (ALD) of zirconia using a self-assembled monolayer (SAM) and atomic force microscopy (AFM) oxidation lithography as a method of fabricating nanostructures. A SAM layer was used as a chemical mask for the ALD process, and AFM oxidation lithography was used as a nanoscale patterning tool. AFM oxidation lithography was applied to create oxide patterns on octadecyltrichlorosilane SAM-grown silicon substrates. Subsequent HF etching removed the oxide patterns locally, exposing a silicon substrate underneath. After 100 cycles of the ALD process, ALD nanostructures high and sub-100 nm wide were fabricated with no detectable Zr element outside the pattern defined by AFM oxidation lithography. The present method makes it possible to construct advanced metal-oxide-semiconductor field effect transistor and quantum confinement structures with a wide range of ALD precursor materials.

Asphaltene Adsorption onto Self-Assembled Monolayers of Alkyltrichlorosilanes of Varying Chain Length

The adsorption of asphaltenes onto flat silica surfaces modified with self-assembled monolayers (SAMs) of alkyltrichlorosilanes of varying thickness due to a variable number of carbon atoms (N(C)) has been studied by means of contact angle measurements, spectroscopic ellipsometry, and near-edge X-ray absorption fine structure spectroscopy. The extent of asphaltene adsorption was found to depend primarily on the ability of the SAM layer to shield the underlying silicon substrate from interacting with the asphaltenes present in solution. Specifically, asphaltene adsorption decreased with an increase in N(C) and/or an increase in SAM grafting density, sigma(SAM), (i.e., number of SAM molecules per unit area). The effect of the solvent quality on the extent of asphaltene adsorption was gauged by adsorbing asphaltenes from toluene, 1-methylnaphthalene, tetralin, decalin, and toluene-heptanes mixtures. The extent of asphaltene adsorption was found to increase proportionally with a decrease in the Hildebrand solubility parameter of the solvent.

Dynamics of the Reaction of O(3P) Atoms with Alkylthiol Self-assembled Monolayers

We have studied the dynamics of the reactions of O((3)P) atoms with alkylthiol self-assembled monolayers (SAMs). Superthermal O((3)P) atoms, with a fairly broad distribution of laboratory-frame kinetic energies (mean = 16 kJ mol(-1), fwhm = 26 kJ mol(-1)), were generated by 355 nm photolysis of NO(2) introduced at a low pressure above the SAM surface. Nascent OH v' = 0 products were detected by laser-induced fluorescence. SAMs of two different alkyl chain lengths, C(6) and C(18), were studied. The existence of SAM layers, and their robustness under our experimental conditions during the relevant measurement period, were confirmed by scanning-tunneling microscopy (STM). Reaction at the SAM surface was verified as the authentic source of the hydroxyl radicals using a perdeuterated C(6)D(13)-SAM sample. The OH appearance profiles as a function of photolysis-probe delay, and the rotational-state distributions at their peaks, were compared with those of liquid squalane (C(30)H(62), 2,6,10,15,19,23-hexamethyltetracosane). The reactivity of the SAMs and of squalane was found to be comparable. We conclude that the O((3)P) atoms must be able to access the more reactive secondary hydrogen atoms along the alkyl chains of the SAMs. We find no perceptible differences in reactivity or product energy disposal between the two SAM chain lengths. Both produce a substantial fraction of the OH with relatively high velocities, which must result from direct, impulsive reaction. There is also a slower component, with velocities consistent with a thermal, trapping-desorption mechanism. The proportion of this component appears to be lower for SAMs than for squalane. This would be compatible with the expected greater smoothness of the SAM surface at the molecular scale. We find little evidence for significant rotational excitation of the OH products, although the details of any correlation between translational and rotational energy release require further investigation. We compare our results with the limited available prior theoretical modeling of O((3)P) + SAM systems.

An Electrochemical Study of 4-Aminothiophenol/Pt Nanoparticle Multilayers on Gold Electrodes

Quartz crystal microbalance (QCM) measurements of the formation of a 4-aminothiophenol (4-ATP) self-assembled monolayer (SAM) at a gold electrode showed that a surface coverage of 118 ng cm(-2) was obtained after a 3 h exposure period, indicating that good surface coverage was achieved. Cyclic voltammetry of the ferricyanide redox couple across this SAM modified surface produced similar results to those of a bare electrode; however, the electroreduction of oxygen was found to be impaired. The 4-ATP SAM layer was not stable to repeated electrochemical oxidation and reduction; it is believed that the 4-ATP SAM layer was first converted to a 4'-mercapto-N-phenylquinone diimine (NPQD) layer followed by subsequent formation of a 4'-mercapto-N-phenylquinone monoimine (NPQM) layer. We also report a quartz crystal microbalance study of the attachment of platinum nanoparticles to such SAM modified electrodes. We show that five times the amount of platinum nanoparticles can be attached to a 4-ATP modified electrode surface (observed frequency change -187 Hz) compared with an NPQD modified electrode surface (observed frequency change -35 Hz). The presence of the platinum particles was confirmed electrochemically by their surface electrochemical properties, which were different from those of the underlying gold electrode. It is believed that this is the first time that such direct evidence of electrochemical communication between platinum nanoparticles and a SAM modified electrode surface has been obtained. It was also shown to be possible to build up multilayer SAM/nanoparticle modified surfaces while maintaining efficient electrochemical communication. Up to three SAM/nanoparticle sandwich layers were constructed.

Oxidative Removal of Self-Assembled Monolayers for Selective Atomic Layer Deposition

Using atomic force microscopy (AFM) lithography and self-assembled monolayer (SAM), selective atomic layer deposition (ALD) of ZrO2 was performed to fabricate nanostructures. SAM layer was used for a chemical mask for ZrO2 ALD process and AFM lithography for a nano-scale patterning method. AFM anodic oxidation process was applied to create oxide patterns on ODTS-coated silicon substrates. Subsequent HF etching locally removes oxide, exposing silicon layer underneath. After 100 cycles of ZrO2 ALD process and an oxygen plasma etching, ZrO2 ALD nanostructure of 4~7 nm in height and ~130 nm in line width were fabricated. A growth rate of 0.4~0.7 Aå per cycle is estimated from the AFM scanning at each step, which is slightly less than the typical ALD growth rates of ZrO2.

Development of a FPW allergy biosensor for human IgE detection by MEMS and cystamine-based SAM technologies

Abstract Conventional allergy detecting kits usually require a large number of expensive testing samples, time-consuming procedures and complex instruments. To overcome these disadvantages, this study presents a high-sensitivity flexural plate-wave (FPW)-based allergy biosensor with integrated cystamine self-assembled monolayer (SAM) for detecting the concentration of immunoglobulin E (IgE) in human serum. The ultrasonic flexural plate-waves were respectively launched and received by two Cr/Au interdigital transducers (IDTs) located on the left and right side of the Si/SiO2/Si3N4/Cr/Au/ZnO floating thin plate. A 4.625-nm height cystamine SAM and a 1.078-nm height glutaraldehyde cross-linking layer were developed for the immobilization of IgE antibody and their surface chemical composition and structural perturbation changes were analyzed by Auger electron spectroscopy (AES) and Fourier transform-infrared (FT-IR) spectroscopy, respectively. Using atomic force microscopy (AFM), this work investigates the molecular topographies of the cystamine SAM, glutaraldehyde, IgE antibody and IgE antigen layers and the number of bonded IgE antibody/antigen pairs. A FPW-based allergy biosensor with very high mass-sensitivity (−8.52 × 107 cm2 g−1) and low operating frequency (8.75 MHz) of human IgE antigen was demonstrated.

Polymer Brushes Grafted to “Passivated” Silicon Substrates Using Click Chemistry

We present herein a versatile method for grafting polymer brushes to passivated silicon surfaces based on the Cu(I)-catalyzed Huisgen 1,3-dipolar cycloaddition (click chemistry) of omega-azido polymers and alkynyl-functionalized silicon substrates. First, the "passivation" of the silicon substrates toward polymer adsorption was performed by the deposition of an alkyne functionalized self-assembled monolayer (SAM). Then, three tailor-made omega-azido linear brush precursors, i.e., PEG-N3, PMMA-N3, and PS-N3 (Mn approximately 20,000 g/mol), were grafted to alkyne-functionalized SAMs via click chemistry in tetrahydrofuran. The SAM, PEG, PMMA, and PS layers were characterized by ellipsometry, scanning probe microscopy, and water contact angle measurements. Results have shown that the grafting process follows the scaling laws developed for polymer brushes, with a significant dependence over the weight fraction of polymer in the grafting solution and the grafting time. The chemical nature of the brushes has only a weak influence on the click chemistry grafting reaction and morphologies observed, yielding polymer brushes with thickness of ca. 6 nm and grafting densities of ca. 0.2 chains/nm2. The examples developed herein have shown that this highly versatile and tunable approach can be extended to the grafting of a wide range of polymer (pseudo-) brushes to silicon substrates without changing the tethering strategy.

Molecular junctions of ∼1 nm device length on self-assembled monolayer modified n- vs. p-GaAs

Fabrication of high-fidelity molecular junctions of 1 nm or less device length is a significant challenge due to penetration of the top contact through the molecular layer. Successively smaller device lengths allow for molecules to more strongly modulate semiconductor surface charge and conductivity for device gating applications; hence, it is of great interest to better understand electron transport over device length scales of ∼1 nm. Molecular devices on GaAs are of interest, since their integration to band-gap engineered nanostructures on GaAs can enable high-sensitivity molecular signal transduction schemes. In this study, the formation of molecular junctions of ∼1 nm device length was investigated. Complexation and electroless deposition (ELD) of copper on self-assembled monolayer (SAM) modified n- and p-GaAs surfaces was used to correlate the effects of variations in SAM length, terminal functional group, and substrate dopant type (n vs. p) on the morphology of ELD Cu and its penetration through the underlying SAM. Electrochemical, electron microscopy and electrical transport characterization of Cu ELD on SAM-modified n-GaAs demonstrates highly selective deposition only on carboxylic acid terminated surfaces, the strong dependence of ELD morphology on SAM order, and absence of Cu penetration through the SAM, for device lengths down to sub-nanometer scales. Cu ELD on SAM-modified p-GaAs surfaces was similar to that of the respective SAM-modified n-GaAs surface, only in those cases where the SAM layer can passivate p-GaAs from corrosion. In other cases such as the shortest SAMs studied herein, corrosion of p-GaAs resulted in the disruption of sub-nanometer molecular junctions.

Interfacial Electronic Properties of Thiophene and Sexithiophene Adsorbed on a Fluorinated Alkanethiol Monolayer

X-ray and ultraviolet photoelectron spectroscopies (XPS and UPS) have been used to study the interfacial electronic properties of thiophene and α-sexithiophene (6T) adsorbed at 130 K in ultrahigh vacuum on top of 1H,1H,2H,2H-perfluorodecanethiol (PFDT) self-assembled monolayers (SAMs) on gold. The binding energy of the F 1s XPS peak decreases following thiophene and 6T adsorption, indicating charge transfer to the fluorinated SAMs. UPS measurements substantiate charge transfer, with the valence features of thiophene and 6T in contact with the monolayer appearing at higher ionization energies compared to thicker layers. The energies of the UPS-measured vacuum levels of 6T deposited on PFDT/Au illustrate the absence of a common vacuum level between the organic layers at the PFDT-6T interface and the presence of a −0.9 eV interface dipole. Similar measurements performed for 6T deposition on self-assembled octadecanethiol (ODT) give a weaker interface dipole of opposite sign (Δ = +0.4 eV). The relatively large value and sign of the 6T/PFDT/Au interface dipole suggest that charge transfer to the PFDT-covered surface results in the formation of dipoles with their negative ends toward the Au surface, in contrast to the 6T-ODT interface. The effects of a SAM layer on X-ray-induced oligomerization, which is known to occur for condensed thiophene, were also investigated. Comparison of the thickness of oligomeric thiophene formed by Mg Kα X-ray irradiation on clean and PFDT-covered gold surfaces demonstrates that a thicker oligomer layer forms on the SAM-covered surface, suggesting that the spacing provided by the SAM reduces quenching of electronic excitations that lead to X-ray-induced oligomerization.