Monoatomic and cluster beam effect on ToF-SIMS spectra of self-assembled monolayers on gold

Self-assembled monolayers represent well-defined systems that is a good model surface to study the effect of primary ion beams used in secondary ion mass spectrometry. The effect of polyatomic primary beams on both aliphatic and aromatic self-assembled monolayers has been studied. In particular, we analysed the variation of the relative secondary ion yield of both substrate metal-cluster (Au_n⁻) in comparison with the molecular ions (M⁻) and clusters (M_xAu_y⁻) by using Bi⁺, Bi₃⁺, Bi₅⁺ beams. Moreover, the differences in the secondary ion generation efficiency are discussed. The main effect of the cluster beams is related to an increased formation of low-mass fragments and to the enhancement of the substrate related gold-clusters. The results show that, at variance of many other cases, the static SIMS of self-assembled monolayers does not benefit of the use of polyatomic primary ions.     

Molecular dynamics simulation study of the nano-wear characteristics of alkanethiol self-assembled monolayers

A molecular dynamics (MD) simulation study of the probe-based nano-lithography of an alkanethiol self-assembled monolayer (SAM) on a metal surface was performed. The motivation of this work was to understand the nano-tribological phenomena of the nano-metric scribing process of alkanethiol molecules and gain insight into the interaction between the probe tip and the SAM-coated surface during the scribing process. The simulation results revealed that the organothiol molecules were displaced and dragged by the probe tip during scribing due to the strong interchain interactions. It was also found that the scribed pattern width was largely dependent on the tip–surface interaction induced by the probe shape rather than the tip–surface contact size. Also, the minimum load for tip–substrate contact changed with the number of molecules that interact with the probe tip. Furthermore, from the investigation of the effect of the scribing speed on the surface-damage characteristics of the chain molecules, it was found that relatively high-speed scribing could induce excessive removal of the SAM molecules from the surface. PACS 02.70.Ns; 31.15.Qg; 81.16.Nd; 68.35.Af     

Azobenzene-functionalized alkanethiols in self-assembled monolayers on gold

Self-assembled monolayers (SAMs) of 4-trifluoromethyl-azobenzene-4′-methyleneoxy-alkanethiols (CF₃– C₆H₄–N=N–C₆H₄–O–(CH₂) _n –SH on (111)-oriented poly-crystalline gold films on mica were examined by X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS). The spectra are analyzed with the help of density-functional-theory calculations of the isolated molecule. Only one doublet is detected in the sulphur 2p spectra of the investigated SAMs, consistent with a thiolate bond of the molecule to the gold surface. The C 1s XP spectra and the corresponding XAS π ^* resonance exhibit a rich structure which is assigned to the carbon atoms in the different chemical surroundings. Comparing XPS binding energies of the azobenzene moiety and calculated initial-state shifts reveals comparable screening of all C 1s core holes. While the carbon 1s XPS binding energy lies below the π ^*-resonance excitation-energy, the reversed order is found comparing core ionization and neutral core excitation of the nitrogen 1s core-hole of the azo group. This surprising difference in core-hole binding energies is interpreted as site-dependent polarization screening and charge transfer among the densely packed aromatic moieties. We propose that a quenching of the optical excitation within the molecular layer is thus one major reason for the low trans to cis photo-isomerization rate of azobenzene in aromatic-aliphatic SAMs.     

An atomic force microscope study of vanadium-benzene sandwich clusters soft-landed on self-assembled monolayers

Multiple-decker vanadium-benzene sandwich clusters V_n(benzene)_n+1 produced by a laser-vaporization synthesis method were soft-landed onto self-assembled monolayers of alkanethiol (C₁₈H-SAM) and fluorinated alkanethiol (C₁₀F-SAM) at 200 K. Noncontact atomic force microscopy has been used to examine the resulting adsorption states of the clusters landed on the SAMs at room temperature. For each SAM substrate, the aggregates of the deposited clusters were observed at the vacancy islands and near the steps of the SAM surface. The result indicates that, at room temperature, the clusters landed on the SAM substrate thermally diffuse on the surface to form columnar-shape islands around the defect sites of the SAM surface.     

Integrity of functional self-assembled monolayers on hydrogen-terminated silicon-on-insulator wafers

Silicon-on-insulator (SOI) wafers are commonly used to design microelectronics, energy conversion, and sensing devices. Thin solid films on the surfaces of SOI wafers have been a subject of numerous studies. However, SOI wafers modified by self-assembled monolayers (SAMs) that can also be used as functional device platforms have been investigated to a much lesser extent. In the present work, tert-butoxycarbonyl (t-boc, (CH₃)₃-C-O-C(O)-)-protected 1-amino-10-undecene monolayers were covalently attached to a H-terminated SOI (1 0 0) surface. The modified wafers were characterized by X-ray photoelectron spectroscopy to confirm the stability of the SAM/Si interface and the integrity of the secondary amine in the SAM. The transmission electron microscopy investigation suggested that this t-boc-protected 1-amino-10-undecene SAM produces atomically flat interface with the 2 μm single crystalline silicon of the SOI wafer, that the SiO_x and both available Si/SiO_x interfaces are preserved, and that the organic monolayers are stable, with apparent thickness of 1.7 nm, which is consistent with the result of the density functional theory modeling of the molecular features within a SAM.     

Atomic-scale X-ray structural analysis of self-assembled monolayers on Silicon

Two related self-assembled monolayers (SAMs), 4-bromostyrene (BrSty) and 4-bromophenylacetylene (BPA), are photochemically grown from solution on to the monohydride-terminated Si(111) surface. The atomic-scale structures of the resulting SAMs are examined by X-ray standing waves (XSW), X-ray reflectivity (XRR), X-ray fluorescence, atomic-force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT). The coverage is 0.5 ML. The results show that in each case the molecule covalently bonds to a single Si T₁ site and stands up-right with a slight molecular tilt of 17^○ that leaves the Br terminal end over a neighboring T₄ site. The Br height is 8.5?Å (BrSty) and 8.6?Å (BPA) above the top surface Si atom. The combined XSW and XRR results rule-out two alternative bonding models predicted by DFT that have the root of the molecule bonded to two neighboring top Si surface atoms. Based on the XSW 111 and 333 coherent fractions, the BPA/Si(111) has a reduced vertical Br distribution width in comparison to BrSty. This greater rigidity in the molecular structure is correlated to a C=C bond at the root.     

ToF-SIMS study on the cleaning methods of Au surface and their effects on the reproducibility of self-assembled monolayers

The production of reproducible self-assembled monolayers (SAMs) is essential to many nano(bio)technology applications. To check the effects of different cleaning methods on a reproducible SAMs formation, the cleaning methods were varied and then used for preparing each SAM. The reproducibility of each SAM was examined by ToF-SIMS analysis along with principal component analysis (PCA). Using what we found to be a superior method of cleaning gold surfaces, alkanethiol SAMs with different terminal groups such as 1-dodecanethiol (DDT), 11-mercaptoundecanoic acid (MUA), 11-mercapto-1-undecanol (MUD) were reproduced. Our statistical results show that reproducible alkanethiol SAMs on a well-cleaned gold surface can be produced within only a few standard deviation percentages obtained from point-to-point and sample-to-sample spectra.     

自组装双硫醇分子膜在交流电场下的介电特性

研究了交流电场下双巯基烷烃硫醇自组装分子膜的阻抗谱.利用汞金属作为衬底,制备出双巯基烷烃硫醇自组装分子膜,并通过交流频谱仪对其进行频谱的扫描.通过实验明确了膜的作用范围为阻抗谱中频部分,并给出相应的等效电路对阻抗谱进行了拟合.同时,根据损耗谱中损耗峰随硫醇碳链原子数的增加而向低频方向移动的现象得出双巯基硫醇Cn(n=3～10)在交流电场下的动能为14～48 meV.     

Step-wise decomposition process of azobenzene self-assembled monolayers

The step-wise decomposition of 4-(12-(dodecyldithio)dodecyloxy)azobenzene (AzoC24) in self-assembled monolayer (SAMs) on Au(1 1 1) was observed by thermal desorption spectroscopy (TDS) and X-ray photoelectron spectroscopy (XPS) under the ultra-high vacuum (UHV) condition. This decomposition process only occurred after the formation of the SAM.The TD spectra clearly showed two steps of thermal decomposition of the azobenzene moiety. At approximately 450 K, fragments of m/e = 77 and 105 were clearly observed. These fragments were decomposed species obtained by the breakage of the C-N bonds of the azobenzene moiety. At about 490 K, other fragment of m/e = 93 assigned to the phenoxy ion was detected. In order to examine the decomposition process, we measured the S 2p and N 1s XPS of the SAM at various temperatures. The results suggest that diazonium moiety is the first to be decomposed and the remaining structure is desorbed together with breakage of C-O bond between the phenoxy moiety and alkyl chain with increasing temperature.     

Chirality-induced spin-selective properties of self-assembled monolayers of DNA on gold

Here we show that self-assembled monolayers on gold of double-stranded DNA oligomers interact with polarized electrons similarly to a strong and oriented magnetic field. The direction of the field for right-handed DNA is away from the substrate. Moreover, the layer shows very high paramagnetic susceptibility. Interestingly, thiolated single-stranded DNA oligomers on gold do not show this effect. The new findings are rationalized based on recent results in which high paramagnetism was measured for diamagnetic films adsorbed on diamagnetic substrates.     

Mixed self-assembled monolayers of Co-porphyrin and n-alkane phosphonates on gold

The present work focuses on the surface immobilization by self-assembly of pure and mixed Co-porphyrin (Co-Porph-PO₃H₂) and n-alkane phosphonic acids (n-C_nH_2n + 1PO₃H₂; n = 4, 5 and 10) from n-butanol solutions on gold substrates. The stability, amount, and packing of the phosphonic molecules attached to the Au (111) surface were investigated by electrochemical reductive desorption studies, and monolayers' thickness was estimated by ellipsometry. The morphological changes induced by the adsorption of n-decane phosphonic acid on gold were analysed by scanning tunnelling microscopy. The redox behavior of Co-Porph-PO₃H₂ SAMs was assessed in organic medium and compared that of Co-Porph-CO₂CH₃ precursor in solution, confirming the self-assembly of the metalloporphyrin molecules. With the purpose of reducing the electrostatic interactions between the porphyrin bulky terminal groups in the SAM, n-C₅H₁₁PO₃H₂ and n-C₁₀H₂₁PO₃H₂ were used to form mixed monolayers with Co-Porph-PO₃H₂ on gold. Intermediate electrochemical desorption potentials regarding those values of pure monolayers, as well as an increase of phosphonate surface density compared to that of Co-Porph-PO₃H₂ SAM, confirm the presence of two-component SAMs, which indicates that porphyrin moieties are diluted in the monolayer. The electrocatalytic activity of the immobilized molecules was demonstrated towards the reduction of molecular oxygen, in acidic medium.     

Molecular gated transistors: Role of self-assembled monolayers

In order to understand the biosensing mechanism of field-effect based biosensors and optimize their performance, the effect of each of its molecular building block must be understood. In this work the gating effect of self-assembled linker molecules on field-effect transistor was studied in detail. We have combined Kelvin probe force microscopy, current-voltage measurements, capacitance-voltage measurements, equivalent circuit modeling and device simulations in order to trace the mechanism of silicon-on-insulator biological field-effect transistors. The measurements were conducted on the widely used linker molecules (3-aminopropyl)-trimethoxysilane (APTMS) and 11-aminoundecyl-triethoxysilane (AUTES), which were self-assembled on ozone activated silicon oxide surface covering the transistor channel. In a dry environment, the work function of the modified silicon oxide decreased by more than 1.5 eV, and the transistor threshold voltage increased by about 30 V following the self-assembly. A detailed analysis indicates that these changes are due to negative induced charges on the top dielectric layer, and an effective dipole due to the polar monolayer. However, the self-assembly did not change the silicon flat-band voltage when in contact with an electrolyte. This is attributed to electrostatic screening by the electrolyte.     

Device applications of self-assembled monolayers and monolayer-protected nanoclusters

The present status of self-assembled monolayers (SAMs) on different surfaces (2D systems) as well as monolayer formation on metallic and semiconducting cluster surfaces (3D SAM) to form monolayer-protected nanoclusters (MPCs) and their assemblies is reviewed briefly. Attention is focused mainly on the potential electronic and photonic applications of SAMs, MPCs and their 2D and 3D structures fabricated using covalent and hydrophobic interactions in contrast to the usual electrostatic assemblies. These examples illustrate the rational use of organic molecules and nanoclusters using the concept of self-assembly, where subtle systems of double tunnel junctions, hetero junctions and single-electron transition devices could be developed based on the structure and chemistry of multifunctional molecules. The tailoring of cluster size and cluster–cluster spacing to reveal interesting transitions in electronic properties is also demonstrated using the low temperature behavior of the 3D network of nanoclusters as an example. These devices are believed to play an important role in the coming years as the chip functions and clock frequencies reach orders of magnitude beyond those extrapolated from Moore’s law.     

Selective imaging of self-assembled monolayers by tunneling microscopy

The properties of thin organic films offer many challenging opportunities for science and technology. A crucial requirement for the advancement of molecular film technology is the selective characterization and modification on an atomic level. Local proximal probes like Scanning Tunneling Microscopy (STM) or Atomic Force Microscopy (AFM) bear certainly the potential for this purpose. So far, however, mainly adsorbed organic molecules lying flat on a smooth substrate have been imaged with near atomic resolution. Here, we demonstrate the ability of STM to selectively image self-assembled monolayers of long-chain molecules (hexanethiol) oriented upright on the substrate Au(111) with molecular resolution. Upon proper choice of the tunneling parameters we can image the molecular head-group anchored at the substrate and/or the molecular tail group.     

Quantitative analysis of mixed self-assembled monolayers using ToF-SIMS

The spacing of chemical functional groups on self-assembled monolayers (SAMs) plays an important role in controlling the density of biomolecules in biochips and biosensors. In this sense, a mixed SAM made of two different terminal groups is a useful organic surface since spacing can be easily controlled by changing a relative mole fraction in a mixture solution. In this study, an acetylene-OCH₂O(EG)₃(CH₂)₁₁S-S(CH₂)₁₁(EG)₃OCH₂O-propene (Eneyne) SAM and mixed SAMs made by a mixture of (S(CH₂)₁₁(EG)₃OCH₂O-acetylene)₂ (Diyne) and (S(CH₂)₁₁(EG)₃OCH₂O-propene)₂ (Diene) were produced on gold substrates and measured by using ToF-SIMS. The secondary ion yield ratio of [Au·S(CH₂)₁₁(EG)₃OCH₂O-acetylene]⁻ to [Au·S(CH₂)₁₁(EG)₃OCH₂O-propene]⁻ was measured for each mixed SAM and plotted as a function of the mole fraction of Diyne to Diene in a SAM solution. The ion yield ratio of a mixed SAM produced from a solution with a mole fraction of 0.5 (i.e., 1:1 mixture) was 0.3, which corresponded well to the ion yield ratio measured from an Eneyne SAM. A time-dependent experiment of Eneyne SAM formation and immersion experiment of Eneyne SAM into Diyne solution or into Diene solution were performed. The relative ion yield ratio of 0.3 was due to a different secondary ion formation and not due to the difference in the amount of adsorbates on the surface, nor to the different binding strengths onto the gold surface. Our study shows that a mixed SAM with well-controlled spacing can be produced and quantified by using the ToF-SIMS technique.     

Electronic origin of disorder and diffusion at a molecule-metal interface: self-assembled monolayers of CH3S on Cu111

The relationship between structure, interfacial electrostatics, bonding, and dynamics of organic molecules on metals is studied using a self-assembled monolayer of methylthiolate, CH3S, on Cu(111). The flat adsorption energy landscape of CH(3)S/Cu(111) results from metal-to-molecule charge redistribution which allows for a high mobility of CH3S. This contributes a nonuniform diffuse background to Bragg scattering, which needs to be considered in diffraction analyses. Ramifications on the interpretation of experimental data and the potential impact on the design of metal-organic interfaces are discussed.     

Synthesis and surface characterization of alkylthioacetyl-capped self-assembled monolayers on gold surface

A novel alkylthioacetyl-capped hydroxyethyl methacrylate monomer and its corresponding homopolymer have been synthesized and characterized. Direct chemisorption of these moieties have been carried out on gold-coated substrate and found to form a strong surface bonding. The surface coverage and the properties of the resultant self-assembled layers have been investigated by multiple surface characterization techniques (i.e. ellipsometry, GA-FTIR, XPS, AFM, and contact angle measurements). These analyses have all confirmed the occurrence of complete chemisorption reactions with typical n-alkanethiol self-assembled characteristics.     

Rheology of self-assembled monolayers on solid-liquid interface oscillating at MHz frequency

The rheology properties (viscosity and elasticity) of chemisorbed soft matters on a solid-liquid interface oscillating at MHz were investigated using a quartz crystal microbalance (QCM). As a chemisorbed soft matter, we employed the self-assembled monolayers (SAMs) of mercapto oligo(ethylene oxide) methyl ethers, HS(CH₂CH₂O)_nCH₃ (n?=?5, 11, 12, 19, 27, 35 and 43), where those molecular weights had unity. The systematical analyses on the basis of the Voight model revealed the relationships of $\eta \propto M_n^{0.180}$ and $\mu \propto M_n^{0.344}$, where η and μ are the viscosity and elasticity of the SAM, and M_n is the molecular weight of HS(CH₂CH₂O)_nCH₃. As a result, we found that the SAM consisting of the oligomer followed the formula of polymer.     

Role of stress on charge transfer through self-assembled alkanethiol monolayers on Au

We have studied charge transfer through alkanethiol molecules self-assembled on Au(111) substrates using interfacial force microscopy. Simultaneous measurement of the tip-substrate current and the normal interfacial force reveals the critical role of tip-film contact. Measurable currents are seen only for tip-applied stresses above about 20 MPa, after which the current rises exponential with stress. We suggest that charge transfer results from stress-induced band-gap states near the Fermi level in these normally highly insulating molecular films.     

Electron-beam exposure of self-assembled monolayers of 10-undecenoic acid

Tapping mode atomic force microscopy and capacitance versus voltage measurements were employed to study the effects of electron-beam exposure on self-assembled monolayers of 10-undecenoic acid. It was established that exposure increases chemical/mechanical stability, resulting in a thicker layer following a solvent treatment designed to remove residual monomers. Electron exposure also reduces the effects of pinholes in the monolayer, thereby improving dielectric quality.