Self-assembled microarray of organic light-emitting diodes using a self-assembled monolayer by microcontact printing

Self-assembled monolayer (SAM) films have attracted immense attention for both fundamental and applied research. A SAM is composed of a large number of molecules with a head group that chemisorbs onto a substrate, a tail group that interacts with the outer surface of the film, and a spacer (backbone) chain group that connects the head and tail groups resulting in a coating. Interactions between spacer groups of different molecules, such as van der Waals forces and/or hydrogen bonding, hasten SAM film formation and contribute to its stability. In this dissertation, SAM and thin films have been formed onto copper and aluminum oxide surfaces by reaction with 1H,1H,2H,2H-perfluorodecyldimethylchlorosilane (PFMS), 1H,1H,2H,2H-perfluorodecyltrichlorosilane (PFTS), 1H,1H,2H,2H-perfluorodecylphosphonic acid (PFDP), octylphosphonic acid (OP), decylphosphonic acid (DP), and octadecylphosphonic acid (ODP). The properties and stability of the films were investigated employing complementary surface analysis techniques: X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), friction force microscopy (FFM), a derivative of AFM, contact angle measurements (CAMs), and Fourier transform infrared reflection/absorption spectroscopy (FT-IRRAS). The perfluoroalkylsilane SAM on Cu is found to be extremely hydrophobic typically having sessile drop static contact angles of more than 130° for pure water and a surface energy of 14 mJ/m2 (mN/m). FFM showed a significant reduction in the adhesive force and friction coefficient of PFMS modified Cu (PFMS/Cu) compared to unmodified Cu. Treatment by exposure to harsh conditions showed that PFMS/Cu SAM can withstand boiling nitric acid (pH=1.8), boiling water, and warm sodium hydroxide (pH=12, 60 °C) solutions for at least 30 minutes. Furthermore, no SAM degradation was observed when PFMS/Cu was exposed to warm nitric acid solution for up to 70 min at 60 °C or 50 min at 80 °C. XPS and FT-IRRAS data reveal a coordination of the PFMS silicon (Si) atom with a cuprate (CuO) molecule present on the oxidized copper substrate. The data give good evidence that the stability of the SAM film on the PFMS modified oxidized Cu surface is largely due to the formation of a siloxy-copper (-Si-O-Cu-) bond via a condensation reaction between silanol (-Si-OH) and copper hydroxide (CuOH). For a PFTS modified Cu surface (PFTS/Cu), the sessile drop static contact angle of pure water has been measured to be more than 125° and the surface energy to be typically less than 16 mJ/m2. Stability tests show that the PFTS/Cu film can survive in boiling pure water for one hour, boiling nitric acid (pH 1.5 or 1.8) for 30 minutes, sodium hydroxide solution (pH 12, 70 °C) for 30 minutes, and autoclave conditions (steam at 134 °C and 3 atmospheres) for 15 minutes. The more commonly used self-assembled monolayer (SAM) modifications of Cu surfaces, e.g. thiol compounds, are significantly less stable than PFTS/Cu. Extremely hydrophobic (low surface energy) and stable PFMS/Cu SAMs and PFTS/Cu films could be useful as corrosion inhibitors in micro/nanoelectronic devices and/or as promoters for anti-wetting, low adhesion surfaces or drop-wise condensation on heat exchange surfaces. XPS analysis confirmed the presence of perfluorinated and non-perfluorinated alkylphosphonate molecules on the PFDP, DP, and ODP SAMs deposited at the aluminum oxide coated silicon (Al/Si) surfaces. The sessile drop static contact angle of pure water on PFDP SAMs was typically more than 130° and on DP and ODP typically more than 125° indicating that the phosphonic acid SAMs reacted with Al samples were very hydrophobic. The surface roughness for PFDP/Al, DP/Al, ODP/Al, and bare Al was approximately 35 nm, as determined by AFM. The surface energy for PFDP/Al was determined to be approximately 11 mJ/m2 by the Zisman plot method compared to 21 mJ/m2 and 20 mJ/m2 for DP/Al and ODP/Al, respectively. PFDP/Al gave the lowest adhesion and friction force while bare Al gave the highest. The adhesion and friction forces for ODP/Al and DP/Al SAMs were in between. ODP, DP, and OP SAMs have been studied in detail on relatively flat aluminum oxide surfaces. The rms surface roughness for ODP/Al, DP/Al, OP/Al, and bare Al was less than 15 nm, as determined by AFM. The sessile drop static contact angle of pure water on ODP/Al and DP/Al was typically more than 115° and on OP/Al typically less than 105°. The surface energy for ODP/Al and DP/Al was determined to be approximately 21 mJ/m2 and 22 mJ/m2, respectively, compared to 26 mJ/m2 for OP/Al. ODP/Al and OP/Al were studied by FFM to better understand their micro-/nano-tribological properties. ODP/Al gave the lowest coefficient of friction values while bare Al gave the highest. The adhesion forces for ODP/Al and OP/Al were comparable. The chemical stability of ODP/Al, PFDP/Al, DP/Al, OP/Al, and PFMS/Al SAMs has been inspected by exposure to warm nitric acid (pH 1.8, 30 min, 60-95 °C). The XPS data and stability against harsh chemical conditions indicate that a type of bond forms between a phosphonic acid (PA) or silane molecule and the oxidized Al surface. Stability tests using warm nitric acid (pH 1.8, 30 min, 60-95 °C) show ODP/Al SAMs to be most stable followed by PFDP/Al, DP/Al, PFMS/Al, and OP/Al SAMs. For PFTS/Al, stability tests demonstrate that modified aluminum is able to survive exposure to warm nitric acid (pH = 1.8, 60 °C, 30 min) indicating some degree of robustness. Hydrophobic, low adhesion, and robust aluminum surfaces have useful applications for micro/nano-electromechanical systems (MEMS/NEMS), such as digital micro-mirror devices (DMDs). These studies are expected to aid in the design and selection of proper lubricants for MEMS/NEMS.

We report the deposition of DNA nanostructures on self-assembled monolayers (SAMs), focusing on the stability of DNA nanostructures on both hydrophilic and hydrophobic SAMs. Our study reveals distinct outcomes based on the nature of the SAMs. DNA nanostructures maintain structural integrity on hydrophilic SAMs, whereas they experience deformation on the most hydrophobic SAMs. Interestingly, the stability of DNA nanostructures is also sensitive to postdeposition washing procedures. The observations shed light on the intricate interplay between the wettability of SAMs and the structural stability of the DNA nanostructures. An empirical trend emerged where increased hydrophobicity is associated with a more severe deformation of DNA nanostructures. This deformation is hypothesized to arise from disrupted hydrogen bonding within DNA nanostructures and is exacerbated by interfacial tension during the drying process. Our study also highlights the potential role of π-π stacking interactions between the DNA bases and the SAMs in stabilizing the DNA nanostructures. Our work expands the type of substrates that can be used for applications of DNA nanotechnology and highlights the need for a comprehensive understanding of the interactions between DNA nanostructures with different surfaces.

Silver nanocubes are self-assembled into unique structures via hydrophobic interactions. The structures could be predesigned by selectively functionalizing the faces of each nanocube with hydrophobic and hydrophilic self-assembled monolayers (SAMs). The cubic lattice shown in this SEM image was formed at an air-water interface from Ag nanocubes whose entire surface had been derivatized with a hydrophobic SAM.

Self-assembled monolayers (SAM) formed on gold or hydroxylated surfaces are well ordered organic surfaces that enable the control of interface property at the molecular scale. Microcontact printing is a simple technique that can pattern the formation of SAM in the plane with dimensions on the micron scale. The combination of SAM and microcontact printing makes a technique useful for studying a variety of fundamental phenomena in bio-interfacial science such as cell adhesion. However, the existence of surface roughness tends to impede the effectiveness of the method in generating the desirable interface properties. Studies have shown that surface roughness significantly changes adhesion of SAM coated material. The current study tries to understand the roughness effects on bio-adhesion of cell-SAM substratum. A current model of cell adhesion considers the effects of nonspecific and specific interactions between adhesion molecules and the substrate assuming an isotropic Gaussian distributed roughness. Simulation results show the change of pull-off force and contact area due to different scales of roughness. The reduction rate of cell adhesion due to roughness and chemical-mechanical factors are then compared to show the significance of roughness effects on SAM adhesion. Results suggest that caution must be taken in quantitative interpretation of protein and cell adsorption strength by using SAM device as a biosensor.

Semiconductor nanowires are an emerging class of materials with great potential for applications in future electronic devices. The small footprint and the large charge-carrier mobilities of nanowires make them potentially useful for applications with high-integration density, but also to replace thin-film transistors in large-area electronics. This thesis investigates the use of wet-chemically grown ZnO nanowires for the fabrication of high-performance, low-voltage field-effect transistors (FETs). The first part of this thesis addresses the electrical characteristics of the wet-chemically grown ZnO nanowires. The as-grown ZnO nanowires are highly conductive due to a large charge-carrier concentration caused by dopants unintentionally incorporated into the ZnO nanowires during the synthesis. This large charge-carrier concentration makes it difficult to modulate the conductivity of the nanowires by an external electric field, so that the as-grown wires are not suitable for FETs. A post-growth annealing step is employed to dramatically reduce the charge-carrier concentration of the nanowires which makes it possible to fabricate FETs with useful characteristics. ZnO-nanowire FETs in a global back-gate geometry based on annealed ZnO nanowires have a transconductance of 300 nS, an on/off current ratio of 107 and a subthrehold slope of 500 mV/decade. The maximum field-effect mobility of the annealed ZnO nanowires is around 50 cm2/Vs. An important requirement to obtain good FET characterisitcs is to minimize the influence of the source and drain contact resistance on the total device resistance. The material used for the source and drain contacts in this thesis is aluminum. It is demonstrated that the use of a plasma treatment in the contact regions prior to the evaporation of the aluminum contacts can greatly influence the contact properties between ZnO and aluminum. An argon-plasma treatment locally increases the charge-carrier concentration in the ZnO. It is shown that the doping effect of the argon-plasma treatment can be exploited to reduce the contact resistance between alumiunm and ZnO and improve the electrical performance of the FETs. In the second part of this thesis, the influence of the ambient air on the electrical characteristics of the ZnO-nanowire FETs is investigated. The electrical conductivity of the FETs is strongly affected by the distinct atmospheric conditions, which makes it difficult to obtain reliable FET characteristics. The potential of a self-assembled monolayer (SAM) based on fluoroalkylphosphonic acid molecules to passivate the ZnO nanowires against the undesirable effects of the ambient and to stabilize the electrical performance of ZnO-nanowire FETs is demonstrated. The stabilizing effect is attributed to the formation of a densely packed, hydrophobic SAM on the surface of ZnO and aluminum oxide. The quality of the hydrophobic SAMs is confirmed by means of water-contact-angle measurements on SAM-covered ZnO single crystals that show a contact angle of more than 110°. The third part of this thesis is dedicated to the fabrication of high-performance ZnO-nanowire FETs with patterned metal top-gate electrodes. A top-gate fabrication process is developed that uses a very thin gate dielectric consisting only of an alkylphosphonic acid SAM that can be deposited from solution. The insulating quality of the SAM is investigated for top-gate FETs that utilize either gold or aluminum for the top-gate electrode. Gold top-gate FETs show a transconductance of 1 µS, an on/off current ratio of 107, and a steep subthreshold slope of 90 mV/decade. The thin gate dielectric makes it possible to operate the gold top-gate FETs at low voltages of 1 V and simultaneously reduces the undesired gate current by more than three orders of magnitude compared to gold top-gate FETs that have been fabricated without a gate dielectric (MESFETs). The gate current of the gold top-gate FETs with SAM gate dielectric is as low as 1 pA for gate-source voltages between -1 V and 0.5 V. When aluminum is used for the top-gate electrode, a hybrid dielectric is formed at the interface between the SAM and the aluminum, consisting of the SAM and a spontaneously formed aluminum oxide layer. Owing to the hybrid gate dielectric, the aluminum top-gate FETs are operated with gate currents below 1 pA for voltages up to 3 V. The top-gate fabrication process does not require temperatures above 160 °C, which makes it possible to fabricate FETs on unconventional substrates that cannot tolerate high temperatures, such as glass or flexible plastics. Integrated circuits on single ZnO nanowires that are fabricated on glass and plastic substrates are demonstrated. The maximum switching frequency of the inverters is 1 MHz.

Compact self-assembled monolayers (SAMs) of perfluorododecyl iodide (I-PFC12) of reproducible thickness (1.2 nm) are shown to form on silicon wafers. The SAMs have a high fluorine content (95%) and convey an extremely low surface energy to the silicon wafers (4.3 mN m-1), lower than previously reported in the literature for perfluorinated monolayers, and stable for over eight weeks. Shorter chain iodo-perfluorinated (I-PFC8) or bromo-perfluorinated molecules (Br-PFC10) led to less dense layers. The monolayers are stable to heating up to 60 °C, with some loss up to 150 °C. The I-PFC12 monolayer increases the work function of silicon wafers from 3.6 V to 4.4 eV, a factor that could be gainfully used in photovoltaic applications. The I-PFC12 monolayers can be transferred into patterns onto silica substrates by micro-contact printing. The NMR data and the reproducible thickness point to an upright halogen bonding interaction between the iodine in I-PFC12 and the surface oxygen on the native silica layer.

Selective deposition of the silver nano-particles using patterned the hydrophobic self-assembled monolayer patterns and zero-residual nano-imprint lithography

Method of oriented and activity-preserved immobilization of biologically active proteins based on concepts of active-site masking and kinetic control was previously reported. We extended our study and found that the surface hydrophobicity had a noticeable effect on the activity of active-site protected immobilized (PIM) Taq DNA polymerase on mixed self-assembled monolayer (SAM) manufactured on Au surface. Hydrophobic SAM was created by using 12-mercaptododecanoic acid and 1-heptanethiol. The resulting Taq DNA polymerase activity was measured by performing PCR amplification and compared with previously reported values acquired with hydrophilic SAM. The maximum activity of immobilized Taq DNA polymerase was achieved at 17.5% of 12-mercaptododecanoic acid and within 90 min of reaction time which are higher than those acquired with hydrophilic SAM; maximum activity at 5% of 12-mercaptododecanoic acid and at 10 min. In order to be applicable to commercial level, immobilized enzyme need to be stable, reusable and storable. PIM Taq DNA polymerase was stably attached to the surface and could be used for as many as 10 PCR runs which were comparable to solution-phase enzyme. Even after 56 days of storage at 4 °C, immobilized enzyme pertained 70% of the initial PIM enzyme activity. These data suggest that PIM Taq DNA polymerase can be used for various commercial applications.

To understand protein adsorption on a surface is very important in bio-related domains of technology and application such as biomaterials, implant biocompatibility, and biosensor technology. (Gray 2004) Self-assembled monolayers (SAMs) are excellent model surfaces for biology and biochemistry because they are stable, highly ordered, easy to prepare, and provide a wide range of organic functionality. (Love et al. 2005) Previous experimental studies (Ostuni et al. 2003; Prime & G.M. Whitesides 1991) have indicated that hydrophobic interactions are the major mechanism for protein adsorption on surfaces, and the dehydration of both the protein and hydrophobic SAMs provides an entropic driving force for protein adsorption. (Ostuni et al. 2003) Nonetheless, there are still many questions needed to be clarified and difficult to be investigated in experiments. Molecular dynamics (MD) simulation is a powerful tool that is able to investigate the atomic details of a molecular system. This paper summarizes the investigations of proteins adsorption on alkanethiol SAMs by means of MD simulations in order to expand the limited information that the experiments can provide. (Hung et al.2006; Hung et al.2010; Hung et al. 2011)

Selective deposition holds promise to simplify next-generation device fabrication and bring down economic cost. In this work, selectively depositing polymers on metal/dielectric patterns was achieved by spin dewetting, a phenomenon that refers to the dewetting of polymers during spin coating. Our strategy utilizes self-assembled monolayers (SAMs) to induce dewetting of polymers over some areas. Line patterns of Cu/SiO2 were investigated. A hydrophobic SAM, octyltrichlorosilane (OTS, Cl3Si–C8H17), was selectively formed on SiO2 in the presence of Cu to render SiO2 non-wettable. During a subsequent spin coating step, polymers dewet from OTS-functionalized SiO2 and coat Cu exclusively. The spin dewetting process is strongly dictated by the spin coating kinetics. A systematic study of the processing conditions revealed strong dependence of polymer film coverage on spin speed, solution concentration, polymer molecular weight, casting solvent, and SAM hydrophobicity.

A method for the spectroscopic characterization of interfacial fluid molecular structure near solid substrates is reported. The thickness and interfacial molecular structure of residual ultrathin D20 films remaining after forced dewetting on alkanethiolate self-assembled monolayers (SAMs) of 11 1-mercaptoundecanoic acid (11-MUA), 11-mercaptoundecanol (11-MUD), and undecanethiol (UDT) on Ag are investigated using ellipsometry and surface Raman spectroscopy. The residual film thickness left after withdrawal is greater on hydrophilic SAMs than on hydrophobic SAMs. This behavior is rationalized on the basis of differing degrees of fluid slip within the interfacial region due to different interfacial molecular structure. The v(O-D) regions of surface Raman spectra clearly indicate unique interfacial molecular properties within these films that differ from bulk D20. Although the residual films are created by shear forces and Marangoni flow at the three-phase line during the forced dewetting process, the nature of the films sampled optically must also be considered from the standpoint of thin film stability after dewetting. Thus, the resulting D20 films exist in vastly different morphologies depending on the nature of the water-SAM interactions. Residual D20 is proposed to exist as small nanodroplets on UDT surfaces due tospontaneous rupture of the film after dewetting. In contrast, on 11-MUD and 11-MUA surfaces, these films exist in a metastable state that retains their conformal nature on the underlying modified surface. Analysis of the peak intensity ratios of the so-called "ice-like" to "liquid-like" v(O-D) modes suggests more ice-like D20 character near 11-MUD surfaces, but more liquid-like character near 11-MUA and UDT surfaces. The creation of residual ultrathin films by forced dewetting is thus demonstrated to be a powerful method for characterizing interfacial molecular structure of fluids near a solid substrate under ambient conditions of temperature and pressure.

We demonstrate a transfer printing method to obtain polarized light emission in organic light-emitting devices (OLEDs). On a rubbed self-assembled monolayer (SAM), a spin-coated liquid crystalline light-emissive polymer is aligned along the rubbing direction because of the anisotropic interfacial intermolecular interaction. Owing to the low surface energy of the SAM surface, the light-emissive layer was easily transferred to a patterned poly(dimethylsiloxane) (PDMS) stamp surface without degrading the ordering. Finally, a polarized light-emissive OLED device was prepared by transferring the patterned light-emissive layer to the charge transport layer of the OLED structure.

The organic semiconductor materials which form Self-Assembled Monolayers (SAMs) on Indium Tin Oxide (ITO) surface have been synthesized and used in Organic Light Emitting Diode (OLED) applications. The synthesized molecules are bonded chemically to the hydrophilic and rough ITO surface and formed a new hydrophobic and smother surface of ITO by the effect of SAM formation. The synthesized SAM molecule is 4″, 4″″-[biphenyl-4,4″-diylbis(phenylimino)]dibiphenyl-4-carboxylic acid (MZ-187) that contains carboxylic acid head group to bond to ITO surface. The formation on ITO surface is analyzed by cyclic voltammetry (CV) and atomic force microscopy (AFM). Two configurations, ITO/SAM(MZ-187)/HTL/EML/LiF/Al (MZ187-OLED) and ITO/HTL/EML/LiF/Al (bare-OLED), are fabricated to investigate the effect of SAM layer on the performance of the OLED device. Current–voltage (I–V) characterization of OLED devices are carried out and the luminescence of the devices are measured by integrating sphere under nitrogen gas in a glow box system. The quantum efficiency is calculated to determine how to affects the SAM layers on OLEDs performance. As a consequence, it is determined that the OLED device containing MZ-187 SAM layer, a similar structure with N,N′-bis(3-methylphenyl)-N,N′-diphenylbenzidine (TPD), is exhibited a better OLED performance compare to the device without SAM layer.

One of the major challenges for nanofabrication, in particular microcontact printing (mu-CP), is the control of molecular diffusion, or "ink spreading", for the creation of nanopatterns with minimized "smudging" at pattern boundaries. In this study, fully atomistic computer simulations were used to measure the impact of naturally occurring domain boundaries on the diffusion of excess alkanethiol ink molecules on printed alkanethiol self-assembled monolayers (SAM). A periodic unit cell containing approximately one million atoms and with a surface area of 56 nm x 55 nm was used to model a hexadecanethiol SAM on Au(111), featuring SAM domain boundaries and a range of concentrations of excess hexadecanethiol ink molecules diffusing on top. This model was simulated for a total of approximately 80 ns of molecular dynamics. The simulations reveal that domain boundaries impede the diffusion of excess ink molecules and can, in some cases, permanently trap excess inks. There is competition between ink spreading and ink trapping, with the ink/SAM interaction strongly dependent on both the ink concentration and the SAM orientation at domain boundaries. SAM defects thus provide potential diffusion barriers for the control of excess ink spreading, and simulations also illustrate atom-scale mechanisms for the repair of damaged areas of the SAM via self-healing. The ability of domain boundaries to trap excess ink molecules is accounted for using an accessible volume argument, and trapping is discussed in relation to experimental efforts to reduce molecular spreading on SAMs for the creation of ultrahigh resolution nanopatterns.

Zein, the prolamine of corn, has been investigated for its potential as an industrial biopolymer. In previous research, zein was plasticized with oleic acid and formed into sheets/films. Physical properties of films were affected by film structure and controlled in turn by zein-oleic acid interactions. The nature of such interactions is not well understood. Thus, protein-fatty acid interactions were investigated in this work by the use of surface plasmon resonance (SPR). Zein adsorption from 75% aqueous 2-propanol solutions, 0.05% to 0.5% w/v, onto hydrophilic and hydrophobic self-assembled monolayers (SAMs) formed by 11-mercaptoundecanoic acid and 1-octanethiol, respectively, was monitored by high time resolution SPR. Initial adsorption rate and ultimate surface coverage increased with bulk protein concentration for both surfaces. The initial slope of plotted adsorption isotherms was higher on 11-mercaptoundecanoic acid than on 1-octanethiol, indicating higher zein affinity for hydrophilic SAMs. Also, maximum adsorption values were higher for zein on hydrophilic than on hydrophobic SAMs. Flushing off loosely bound zein in the SPR cell allowed estimation of apparent monolayer values. Differences in monolayer values for hydrophobic and hydrophilic surfaces were explained in terms of zein adsorption footprint.