Close-packed Monolayer Research Articles

Rapid nanopatterning technique based on monolayer silica nanosphere close-packing by spin coating

Nanoparticles monolayer formation by spin coating is considered to be a simple, fast and inexpensive nanopatterning technique. However, the parameters that govern the overall growth process in this technique are not completely quantified and techniques for the controlled and continuous growth of close packed monolayer particle arrays without defects need to be developed. In this paper, an ordered particle array formation process is analyzed theoretically, employing material flux balance and particle-subjected forces balance, based on the film thickness model of spin coating and evaporation rate law. A series of experiments were conducted using silica particle suspensions with various particle volume fractions and different spin speeds. The results show that the spin speed should match the particle volume fraction to meet the requirements of material flux and particles movement in order to obtain a close packed monolayer film. The formation mechanism of fabrication defects involving particle agglomeration and uncontrollable voids were analyzed qualitatively based on crystal growth theory, and validation experiments were performed. The formation of highly uniform close-packed monolayer films was demonstrated and the condition requirements for achieving monolayer nanoparticles array with good quality presented.

Resistive Sensors Based on Self-Assembled Core-Shell Nanoparticles

A gas sensor allows the detection thanks to the specific reactivity of its sensitive material towards the target gas. The use of resistive sensors is based on measuring the variation of their electrical conductivity when the target gas is in contact with the sensitive material. In our study, we chose to work with self assembled core-shell nanoparticles as sensing materials. The idea consists on decorating spherical gold nanoparticles, insensitive to the target gases, with a shell based on a different material. The latter provides the specific sensitivity to the sensor. Previous works in our laboratory focused on Au@Pd and Au@Pt nanoparticles as H2 sensors. These works were extended to other shell materials to examine their sensitivity towards H2 as well as other target gases including NH3. Au@ZnO, Au@SnO2 and Au@Ag nanoparticles were synthetized and then assembled by Langmuir Blodgett technique. The assembly resulted in close-packed core-shell nanoparticle monolayers. These layers were transferred, by simple dip-coating, to a glass slide supporting interdigitated electrodes. Sensing performances of the as fabricated sensors were evaluated through the resistivity changes in the discontinuous particle assemble. The sensitivity of Au@ZnO and Au@SnO2 based sensors was tested towards H2 while Au@Ag based sensors were tested under NH3. All sensors showed responses towards the target gases. The effect of humidity and the sensing temperature on the sensing performances were studied. Besides, among the main sensor proprieties requested and studied we note : sensitivity, selectivity, response and recovery times. Assembles of core-shell nanoparticles organized with Langmuir-Blodgett technique demonstrated attractive sensing performances towards H2 and NH3 in extended concentration ranges. Another important contribution concerns the elucidation of the sensing mechanisms. Various analytical techniques including Temperature Programmed Desorption, Temperature Programmed Reduction and Microcalorimetry were used to allow the discussion of the sensing performances towards mechanisms. Figure 1

Resistive Sensors Based on Self-Assembled Core-Shell Nanoparticles

Au@Ag, Au@ZnO and Au@SnO2 nanoparticles were synthetized and then assembled by Langmuir Blodgett technique. The resulting close-packed core-shell nanoparticle monolayers were transferred onto glass chips to fabricate NH3 and H2 resistive sensors. Sensing performances of the as fabricated sensors were evaluated through the resistivity changes in the discontinuous particle assemble. The sensitivity of Au@Ag based sensor was tested towards NH3 while Au@ZnO and Au@SnO2 based sensors were tested under H2. Assembles of core-shell nanoparticles organized by the Langmuir-Blodgett technique demonstrated attractive sensing performances towards the target gases in extended concentration ranges.

Effect of time and deposition method on quality of phosphonic acid modifier self-assembled monolayers on indium zinc oxide

Phosphonic acid (PA) self-assembled monolayers (SAMs) are utilized at critical interfaces between transparent conductive oxides (TCO) and organic active layers in organic photovoltaic devices (OPVs). The effects of PA deposition method and time on the formation of close-packed, high-quality monolayers is investigated here for SAMs fabricated by solution deposition, micro-contact printing, and spray coating. The solution deposition isotherm for pentafluorinated benzylphosphonic acid (F5BnPA) on indium-doped zinc oxide (IZO) is studied using polarization modulation-infrared reflection-absorption spectroscopy (PM-IRRAS) at room temperature as a model PA/IZO system. Fast surface adsorption occurs within the first min; however, well-oriented high-quality SAMs are reached only after ∼48h, presumably through a continual process of molecular adsorption/desorption and monolayer filling accompanied by molecular reorientation. Two other rapid, soak-free deposition techniques, micro-contact printing and spray coating, are also explored. SAM quality is compared for deposition of phenyl phosphonic acid (PPA), F13-octylphosphonic acid (F13OPA), and pentafluorinated benzyl phosphonic acid (F5BnPA) by solution deposition, micro-contact printing and spray coating using PM-IRRAS. In contrast to micro-contact printing and spray coating techniques, 48–168h solution deposition at both room temperature and 70°C result in contamination- and surface etch-free close-packed monolayers with good reproducibility. SAMs fabricated by micro-contact printing and spray coating are much less well ordered.

Observing single-atom diffusion at a molecule-metal interface

The dynamics at the interface between a close-packed porphyrin monolayer and Au(111) is investigated by time-dependent scanning tunneling microscopy, detecting the motion of single-interface adatoms in real space. Imaging sequences reveal predominant switching of the molecular appearance in adjacent molecules, pointing to a spatial correlation that is consistent with adatom diffusion from one molecule to the next. In some cases, the number of switching molecules is drastically increased, indicating collective switching events. In addition to the thermally induced motion of adatoms at the interface, also voltage pulses from the microscope tip can induce the process---revealing different yields in agreement with the model of adatom hopping.

Transient Monolayer Structure of Rubrene on Graphite: Impact on Hole–Phonon Coupling

Charge transport in molecular thin films is often dominated by incoherent hopping processes, and charge-carrier phonon coupling plays a major role in defining mobilities. Our high resolution angle-resolved ultraviolet photoelectron spectroscopy (ARUPS) study reveals the influence of molecular configuration and packing on the hole-phonon coupling in vacuum-sublimed thin films of rubrene on graphite and allows determining charge reorganization energies. In the contact layer to the substrate rubrene is well-ordered with, for low coverages, the tetracene backbone being almost parallel to the graphite surface forming a loose-packed monolayer. Increasing the coverage leads to an orientational transition and a more tilted orientation of rubrene in a close-packed monolayer. This transition results in dramatic changes of spectral features in ARUPS including the photoelectron angular distribution of the highest occupied molecular orbital derived intensity. The charge reorganization energy, however, only changes sli...

Self-assembly of monodisperse composite microgels with bimetallic nanorods as core and PNIPAM as shell into close-packed monolayers and SERS efficiency

In this study, monolayer film of composite microgels with bimetallic nanorods as core and PNIPAM as shell (Au@AgNR@PNIPAM) assembly on a modified surface of silicon wafer was formed by a dip coating method. The effects of temperature, pH and concentration of KCl on the surface morphology of monolayer were discussed. And when the temperature was 50°C; pH value was 4.0; concentration of KCl was 10mM, a monolayer film would be formed much better. By the characterization of atomic force microscopy (AFM) and field emission scanning electron microscopy (FE-SEM), the monolayer structure of Au@AgNR@PNIPAM composite microgels assembled products was confirmed. The analysis of attenuated total reflection Fourier transform infrared absorption spectrum (ATR-FTIR) confirmed that the chemical composition of monolayer film on the silicon wafer surface was PNIPAM. And the UV–visible spectroscopy showed Au@AgNR in the monolayer film still maintained their LSPR optical performance. Assembled product has high stability, the surface morphology and coverage of the monolayer film did not change significantly when testing in a swing speed of 100rpm Rocking Shaker within 30min. The assembled monolayer was used as SERS substrates to trace the 10−1M to 10−4M 1-naphthol in aqueous solution, and the analysis results was with good reproducibility.

The activation mechanism of Cu(II) to ilmenite and subsequent flotation response to α-hydroxyoctyl phosphinic acid

In order to get close-packed hydrophobic molecular monolayer, surface modification of ilmenite was carried out with cupric ions. As it was found by zeta potential and XPS measurement, the cupric ions modification occurs via ion exchange, metal hydroxides adsorption, and redox processes. After modifying by cupric ions, ilmenite surface properties became more active and the adsorption amount of α-hydroxyoctyl phosphinic acid (HPA) increased. As expected, the flotation behaviour of ilmenite improved. XPS measurements verified the ability to establish a covalently attached HPA on the surfaces of ilmenite after Cu(II) activation. The findings of this study will provide a fundamental basis for strengthening flotation.

Resistive switching characteristics in manganese oxide and tantalum oxide devices

The monodisperse manganese oxide nanoparticles with an average diameter of 30nm were chemically synthesized. The nanoparticles assembled as a close-packed monolayer on Pt bottom electrode by dip-coating and annealing process. The Ta/Ta2O5/MnO/Pt device was fabricated. The bipolar resistive switching behaviors could be caused by the formation and rupture of conductive filaments in the switching layers. The stable self-compliance property was demonstrated which can be attributed to the high resistance Ta/Ta2O5 interface, the Schottky barrier of Ta/Ta2O5, and the discontinuity of conduction band at Ta2O5 and MnO interface. The retention characteristics of Ta/Ta2O5/MnO/Pt device were investigated. The conduction mechanisms of Ohmic conduction, space charge limited conduction, Schottky conduction and Poole–Frenkel emission had been investigated for resistance switching mechanism.

A SERS and electrical sensor from gas-phase generated Ag nanoparticles self-assembled on planar substrates.

Optical excitation of coupled plasmonic nanoparticles supports intense localized electromagnetic "hot-spots" which enable a variety of surface enhanced spectroscopies with the best known example being surface enhanced Raman scattering (SERS), currently of great interest for sensing applications. In this study, we present a novel SERS and electrical dual transduction chemical sensor based on gas-phase generated, negatively charged, silver nanoparticles self-assembled on glass slide forming a close-packed plasmonic monolayer thin-film that supports both SERS and electrical sensing. We demonstrate broad tunability of the localized surface plasmon resonance (LSPR) of the close-packed plasmonic nanoparticle monolayer thin-film sensors through control of the nanoparticle (NP) deposition time which directly influences the plasmonic coupling between neghibouring NPs. This broad tunability supports strong SERS activity from visible to near infrared (NIR) excitation wavelengths. We performed SERS and electrical measurements of a non-resonant molecule 4-mercaptobenzonitrile (4-MBN) as a sample Raman reporter molecule to determine the SERS enhancement factor of our SERS substrate. We measured an average SERS enhancement factor of 10(7) from our close-packed plasmonic nanoparticle monolayer thin-film sensor. Films which were grown below or above one nanoparticle monolayer both exhibited significantly lower SERS performance in one or more of SERS enhancement factor (EF), uniformity or repeatability. Our close-packed plasmonic nanoparticle monolayer thin-film sensors are highly uniform from point-to-point across the entire substrate and showed good reproducibility from batch-to-batch. These qualities are highly desirable for quantifiable detection of chemical and biological molecules. As an example application, this type of substrates provides an affordable and reliable sensing and identification capability for combatting new and emerging chemical and biological threats in support of security applications.

Organic Monolayer Protected Topological Surface State.

Perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA)/Bi2Se3 and Fe/PTCDA/Bi2Se3 heterointerfaces are investigated using scanning tunneling microscopy and spectroscopy. The close-packed self-assembled PTCDA monolayer possesses big molecular band gap and weak molecule-substrate interactions, which leaves the Bi2Se3 topological surface state intact under PTCDA. Formation of Fe-PTCDA hybrids removes interactions between the Fe dopant and the Bi2Se3 surface, such as doping effects and Coulomb scattering. Our findings reveal the functionality of PTCDA to prevent dopant disturbances in the TSS and provide an effective alternative for interface designs of realistic TI devices.

Characterization of protein adsorption onto silica nanoparticles: influence of pH and ionic strength.

The adsorption of lysozyme and ß-lactoglobulin onto silica nanoparticles (diameter 21 nm) was studied in the pH range 2–11 at three different ionic strengths. Since the two proteins have a widely different isoelectric point (pI), electrostatic interactions with the negative silica surface lead to a different dependence of adsorption on pH. For lysozyme (pI ≈ 11), the adsorption level increases with pH and reaches a value corresponding to about two close-packed monolayers at pH = pI. In the multilayer adsorption region near pI, added electrolyte causes a decrease in adsorption, which is attributed to the screening of attractive interactions between protein molecules in the first and second adsorbed layer. For ß-lactoglobulin (pI ≈ 5), a pronounced maximum of the adsorbed amount is found at pH 4 in the absence of salt. It is attributed to the adsorption of oligomers of the protein that exist in the solution at this pH. An inversion in the influence of salt on the adsorbed amount occurs at pH > pI, where the protein and the surface are both negatively charged. This inversion is attributed to the screening of the repulsive protein-surface and protein–protein interactions. The adsorption isotherms were analyzed with the Guggenheim–Anderson–De Boer (GAB) model, which allows for two adsorption states (strongly and weakly bound protein).

Substrate effect of laser surface sub-micro patterning by means of self-assembly SiO2 microsphere array

Close-packed monolayer silica spheres organized through a self-assembly process have been widely used in micro/nano fabrications. In this study, we present the effects of substrate on the near-field enhancement of microsphere-assisted laser processing. There are significant differences in the surface morphology of sub-micro hole arrays on substrates with different refractive indices, which were fabricated at the same laser fluence by a KrF excimer laser (λ=248nm). It is found that the position of the maximum enhancement removes from the exit surface of the silica sphere into the sphere as the refractive index of substrate increased based on the simulation. In addition, the field distribution curve oscillates when there is a substrate under the microsphere, and oscillation spreads deeper into the substrate with a high refractive index. The results contribute toward the optimization of the process conditions of the microsphere-assisted laser micro/nano patterning technique and make the modification profile of the material more controllable.

An efficient and surface-benign purification scheme for colloidal nanocrystals based on quantitative assessment

General application of “greener methods” to the synthesis of monodisperse colloidal nanocrystals introduces impurities, including metal carboxylate precursors, non-volatile solvents, free ligands, and non-nanocrystalline side products. These impurities seriously diminish the solution processability and potential applications of colloidal nanocrystals. A protocol was established for evaluating purification schemes. The results revealed that commonly applied purification schemes and their variants do not exhibit a high level of performance and may degrade the ligand surface coverage. A new scheme involving chloroform–acetonitrile precipitation quantitatively removed all impurities from colloidal solutions of CdSe and CdS nanocrystals coated with a variety of carboxylate ligands. The new scheme was benign to the surface structure of nanocrystal-ligands complexes and resulted in each nanocrystal bearing a close-packed monolayer of carboxylate ligands.

Self-assembled C60 layers on incommensurate Cu/Si(111)‘pseudo-5 × 5’ surface

Self-assembled growth of C60 monolayers on the pristine and Ge-modified Cu/Si(111)‘pseudo-5×5’ surfaces was studied using scanning tunneling microscopy observations. The pristine Cu/Si(111)‘pseudo-5×5’ surface shows up as an incommensurate reconstruction on Si(111) substrate, while Ge-modified surface shows up as an array of Ge atomic clusters grown on the Cu/Si(111)‘pseudo-5×5’ template. It has been found that C60 fullerenes are highly mobile on the both surfaces, hence at the early growth stages fullerenes are accumulated along the atomic steps forming their quasi-one-dimensional molecular stripes. With further C60 deposition, almost ideal two-dimensional close-packed molecular monolayers are formed. The layers are modulated as evidenced by developing quasi-periodic pattern of dim and bright fullerenes displaying 2×2 periodicity. Contrast difference between dim and bright fullerenes is supposed to have a topographical origin, namely, bright fullerenes reside higher than dim fullerenes. Dim fullerenes were concluded to occupy centers of hexagons which constitute honeycomb-like structure of the Cu/Si(111)‘pseudo-5×5’ surface. For the Ge-modified surface, this means that adsorbing C60 fullerenes displace Ge atoms from their original positions to the interstitial sites in the molecular monolayer. Ge atoms were found to terminate rotational motion of selected fullerenes in the layer.

Interfacial and intermolecular interactions determining the rotational orientation of C60 adsorbed on Au(111)

Close-packed monolayers of fullerenes on metallic substrates are very rich systems with respect to their rotational degrees of freedom and possible interactions with different adsorption sites or next neighbours. In this connection, we report in detail on the (2√3×2√3)R30°-superstructure of C60 with respect to the Au(111)-surface. We use molecular orbital imaging in systematic UHV-STM studies to reveal the delicate balance of interfacial and intermolecular interactions in this system. Thus, bright C60-molecules in 5:6-top and 6:6-top geometries are observed depending on the respective next neighbours. Moreover, tiny changes in the appearance of the unoccupied molecular orbitals of dim C60-molecules in hex-vac positions are identified which are caused by the respective interaction with the facets surrounding the Au-vacancy.

Effect of packing density and packing geometry on light extraction of III-nitride light-emitting diodes with microsphere arrays

The finite-difference time-domain method was employed to calculate light extraction efficiency of thin-film flip-chip InGaN/GaN quantum well light-emitting diodes (LEDs) with TiO2 microsphere arrays. The extraction efficiency for LEDs with microsphere arrays was investigated by focusing on the effect of the packing density, packing configuration, and diameter-to-period ratio. The comparison studies revealed the importance of having a hexagonal and close-packed monolayer microsphere array configuration for achieving optimum extraction efficiency, which translated into a 3.6-fold enhancement in light extraction compared to that for a planar LED. This improvement is attributed to the reduced Fresnel reflection and enlarged light escape cone. The engineering of the far-field radiation patterns was also demonstrated by tuning the packing density and packing configuration of the microsphere arrays.

In-Situ Stress Measurements during Cobalt Electrodeposition on (111)-Textured Au

The electrodeposition of Cobalt1-3 thin films has attracted much attention in recent years owing to its uses in both magnetic recording as well as alkaline fuel cell catalysis. The evolution of residual stress during thin film growth is complex, having dependence on the microstructure, growth conditions, and material parameters. However, numerous studies have led to a better understanding and mechanisms of residual stress are beginning to emerge. During film deposition, stress is small in the initial stages, eventually becoming tensile during the coalescence of nuclei into a continuous film. However, as films become increasingly thicker and the height becomes more uniform, the stress often assumes a constant steady-state value. Models have been developed that treat this steady state stress as a competition between the tensile and compressive stress generation mechanisms that are governed by atomic mobility, microstructure, and deposition rate4. In this study, the stress was measured with optical wafer curvature techniques that can be used during electrodeposition to determine real-time stress. The measured curvature is proportional to the product of the average biaxial stress (stress-thickness product) and the film thickness vis-à-vis the Stoney equation. Figure 1 shows a cyclic voltammogram (CV)/ stress thickness curve for the deposition of Co on (111) textured Au in 0.1 M NaClO4 + 0.001 M Co(ClO4)2. The CV shows a small peak at -1.0 V, due to the reduction of H+ on the Au surface followed by the reduction of Co at -1.2 V, with increasing overpotential eventually leading to HER due to H2O reduction. The anodic crossover on the return sweep occurs at a potential of -1.08 V, which is presumably close to the equilibrium value of -1.03V. The accompanying stress curve shows that prior to Co reduction, there is a slight increase in the tensile stress owing to electrocapillarity as well as the desorption of ClO4 - from the Au surface. Upon the initial reduction of Co, the stress initially becomes compressive (about -0.2 N/m) before turning back to the tensile direction. The negative electric charge associated with the peak in the compressive stress is about 500 µC/cm2. Considering a 2-electron reduction process, this corresponds to less than 1 monolayer of cobalt metal, assuming that 590 µC/cm2 is required to deposit an incommensurate close-packed monolayer The stress-thickness reaches a maximum of ~ +1.5 N/m under cathodic scan. It is interesting to note that little stress change is observed at potentials more negative than -1.4 V, reflecting the very low faradaic efficiency for Co deposition once H2O is able to be reduced. On the reverse sweep, additional tensile stress (+3.25 N/m) is produced until dissolution of Co occurs further along the anodic cycle. This talk will examine the nature of the stress generation in Cobalt electrodeposited films with focus on overpotential and structure.

Oriented colloidal-crystal thin films of polystyrene spheres via spin coating

We developed a simple and inexpensive synthesis of a large-scale close-packed monolayer of polystyrene sphere arrays, which have a variety of applications. The influence of three step spin speeds, spinning time, solution quantity and relative humidity is studied in order to achieve a large area close-packed monolayer. A relatively high surface coverage and uniform monolayer of PS spheres in the range of 85%–90% are achieved by appropriate control of the preparative parameters. Also the effect of the oxygen plasma etching process on the reduction of PS spheres has been studied. We conclude that it can be useful in industrial applications, because of the fabrication speed, surface coverage, control over PS spheres and cost of the process.

Two Dimensional Aggregation Behaviors of Quinoxaline Dendrimers.

This study focuses on the molecular behavior of two dendrimers containing a hydrophilic core group (carboxyl group) and hydrophobic branches (quinoxaline and methoxyphenyl groups), 2,3-bis(4-(2,3- bis(4-methoxyphenyl)quinoxalin-6-yloxy)phenyl)quinoxaline-6-carb-oxylic acid (G2) and 2,3-bis(4-(2,3-bis(4-(2,3-bis(4-methoxyphenyl)quinoxalin-6-yloxy)phe-nyl)quinoxalin-6-y-oxy)phenyl) quin oxaline-6-carboxylic acid (G3) at the air-water interface. To understand the mechanism of the self-assembly of these molecules, we measured the surface pressure-area (III-A) isotherm and investigated the surface morphology of Langmuir-Blodgett films transferred onto hydrophilic silicon wafers using atomic force microscopy (AFM). Upon compression, G2 molecules stand up and steadily make close-packed monolayer whereas G3 molecules form circular domains and gradually make aggregates of domains. These results were confirmed by the X-ray Reflectivity (XRR) profiles of G2 and G3 monolayers transferred onto silicon substrates.