AFM Topography Research Articles

Surface morphology of PMMA/boehmite hybrid nanostructures prepared via facile one-pot process

In this study, we developed the novel aqueous-based PMMA/AlOOH hybrid by a one-pot process starting from their relevant precursors in a controlled manner. Starting chemical reactions directly from precursors and the sequence of adding reactants provide the possibility towards enhancing the homogeneity of the final product. Inorganic and organic segments were made compatible by means of oleic acid as a coupling agent. Boehmite to PMMA weight ratio as the main parameter was varied from 0 to 18% and the morphology, particle size, size distribution, and topography map of hybrids was shown to be composition dependent. Final PMMA/Bo nanohybrids were characterized using FTIR to confirm the chemical interactions between inorganic and organic segments. TEM analysis showed that nanohybrid particles with irregular shapes containing inorganic particles dispersed in the organic matrix are formed with an average diameter which depends on boehmite content. The presence of phase transformation of Bo makes PMMA/Bo hybrids significantly thermally stable. According to AFM topography map analysis and relevant Gaussian fit function, the roughness of nanocomposite, the size of hybrid nanoparticles and deviation from the mean value (size) were increased as Bo increases from 0 to 18.

The biochemical, nanomechanical and chemometric signatures of brain cancer

Raman spectroscopy and imaging combined with AFM topography and mechanical indentation by AFM have been shown to be an effective tool for analysis and discrimination of human brain tumors from normal structures. Raman methods have potential to be applied in clinical practice as they allow for identification of tumor margins during surgery. In this study, we investigate medulloblastoma (grade IV WHO) (n=5) and the tissue from the negative margins used as normal controls. We compare a high grade medulloblastoma (IV grade), and non-tumor samples from human central nervous system (CNS) tissue. Based on the properties of the Raman vibrational spectra and Raman images we provide a real–time feedback that is label-free method to monitor tumor metabolism that reveals reprogramming of biosynthesis of lipids, and proteins. We have found that the high-grade tumors of central nervous system (medulloblastoma) exhibit enhanced level of β-sheet conformation and down-regulated level of α-helix conformation when comparing against normal tissue. We have shown that the ratio of Raman intensities I2930/I2845 at 2930 and 2845cm−1 is a good source of information on the ratio of lipid and protein contents. We have found that the ratio reflects the lipid and protein contents of tumorous brain tissue compared to the non-tumor tissue. Almost all brain tumors have the Raman intensity ratios significantly higher (1.99±0.026) than that found in non-tumor brain tissue, which is 1.456±0.02, and indicates that the relative amount of lipids compared to proteins is significantly higher in the normal brain tissue. Mechanical indentation using AFM on sliced human brain tissues (medulloblastoma, grade IV) revealed that the mechanical properties of this tissue are strongly heterogeneous, between 1.8 and 75.7kPa, and the mean of 27.16kPa. The sensitivity and specificity obtained directly from PLSDA and cross validation gives a sensitivity and specificity of 98.5% and 96% and 96.3% and 92% for cross-validation, respectively. The high sensitivity and specificity demonstrates usefulness for a proper decision for a Raman diagnostic test on biochemical alterations monitored by Raman spectroscopy related to brain cancer development.

AFM tip-sample convolution effects for cylinder protrusions

A thorough understanding about the AFM tip geometry dependent artifacts and tip-sample convolution effect is essential for reliable AFM topographic characterization and dimensional metrology. Using rigid sapphire cylinder protrusions (diameter: 2.25μm, height: 575nm) as the model system, a systematic and quantitative study about the imaging artifacts of four types of tips—two different pyramidal tips, one tetrahedral tip and one super sharp whisker tip—is carried out through comparing tip geometry dependent variations in AFM topography of cylinders and constructing the rigid tip-cylinder convolution models. We found that the imaging artifacts and the tip-sample convolution effect are critically related to the actual inclination of the working cantilever, the tip geometry, and the obstructive contacts between the working tip’s planes/edges and the cylinder. Artifact-free images can only be obtained provided that all planes and edges of the working tip are steeper than the cylinder sidewalls. The findings reported here will contribute to reliable AFM characterization of surface features of micron or hundreds of nanometers in height that are frequently met in semiconductor, biology and materials fields.

Structure, morphology and Raman and optical spectroscopic analysis of In1−xCuxP thin films grown by MOCVD technique for solar cell applications

Nanocrystalline In1−xCuxP thin films (0≤x≤0.5) have been deposited on quartz substrates by a Metal-Organic Chemical Vapor Deposition (MOCVD) technique. The effect of the copper ion content on the structural crystal lattice, morphology and optical behavior of the InP thin films was assessed using X-ray diffraction, scanning electron microscopy, atomic force microscopy, Raman spectroscopy and spectrophotometry. All films exhibited a crystalline cubic zinc blende structure, inferring the solubility of the Cu atoms in the InP crystal structure. The XRD patterns demonstrated that the inclusion of Cu atoms into the InP films forced the nanoparticles in the films to grow along the (111) direction. The AFM topography showed that the Cu ions reduce the surface roughness of deposited films. The Raman spectra of the deposited films contain the first and second order anti-stoke ΓTO, ΓLO, ΧLO+ΧTO, 2ΓTO, and ΓLO+ΓTO bands which are characteristic of the InP crystalline structure. The intensities of these bands decreased with increasing the content of the Cu atoms in the InP crystals implying the creation of a stacking fault density in the InP crystal structure. The In1−xCuxP thin films have shown high optical transparency of 90%. An increase of the optical band gap from 1.38eV to 1.6eV was assigned to the increase of the amount of Cu ions in the InP films. The In0.5Cu0.5P thin film exhibited remarkable optical conductivity with very low dissipation factor which makes it a promising buffer window for solar energy applications.

Annealing and structural properties of composite films

The composite films were investigated by AFM methods before and after annealing. Topographic and phase-contrast AFM images of the composite films at different annealing temperature were obtained. The separate metal granules and larger-scale labyrinth-like formations were described. These formations appear by the process of the film growth, also by film annealing. Strong changes of the structural properties of the films are observed after the percolation transition. The significant changes of the structural properties are connected with nanostructural transformations in the metal granules topology and presence of metal crystal phase.

Synthesis, characterization, electrochemical and surface morphology properties of poly(azomethine-ester)s

In this study, poly(azomethine-ester)s (PAEs) were synthesized via elimination reactions of aromatic dihydroxy compounds containing imine bonding with terephthaloyl chloride. The structures of PAEs containing different aliphatic chains and ether bridges were confirmed by FT-IR, 1H, 13C-NMR and UV-Vis measurements. Electrochemical parameters of the synthesized SBs and PAEs were determined by cyclic voltammetry measurements. Thermal properties of the synthesized monomers and polymers were investigated by TG–DTA, DMA and DSC techniques. Molecular mass distributions of PAEs are determined with gel permeation chromatography measurements. Morphological and topographic properties of the SBs and PAEs containing azomethine linkage in the main chain were investigated by SEM and AFM techniques, respectively. Maximum particle sizes on surfaces of polymers were calculated with AFM analyses. Surface modifications of poly(azomethine-ester)s and maximum particle size on their surface were studied with topographic and 3D AFM images and particular size curves.

Conductive AFM Topography of Intrinsic Conductivity Variations in Silica Based Dielectrics for Memory Applications

As flash memory approaches its scaling limit, resistive RAM (ReRAM) devices are showing promise in providing the next generation of electronic memories. This comes about as a result of their simple structure, small size and low operating voltages, enabling denser device packing and lower power consumption. Many materials have shown desirable properties as thin film active layers in ReRAM architectures, yet silicon based memories allow for facile integration into existing CMOS infrastructures, making them a particularly suitable class of devices for development. We have looked specifically at devices containing amorphous silicon suboxide or hydrogen silsesquioxane (HSQ) active layers sandwiched between a number of different electrode materials, including titanium nitride and platinum. Here, we present recent characterisation results in which we have probed the three-dimensional structure of conducting filaments in our silicon memories. These filaments govern the switching mechanism that gives dielectric materials their memory function and are thought to be composed of oxygen vacancies, generated as the device is subjected to an electric field. Such features are difficult to image with conventional topographical and cross-sectioning techniques, owing to their low contrast against the background of pristine material. Our results demonstrate the importance of conductive atomic force microscopy for three-dimensional analyses and show that the filament formation is closely linked to the pristine structure of the dielectric. Growth pathways conform to the intrinsic inhomogeneities of the films, a feature that is thought to produce defect clustering and thus promote the generation of oxygen vacancies under electrical stress. We have also been able to demonstrate, for the first time in an intrinsic switching material, that multiple growths compete to bridge the active layer while only a single pathway is finally successful.

Connection Between Atomic Scale Characterization and Electrochemical Behavior during Passivation of Single Crystals on Ni-Cr and Ni-Cr-Mo Alloys

Ni-Cr and Ni-Cr-Mo alloys owe their outstanding corrosion resistance to the surface enrichment of passivating Cr(III) oxides and synergistic effect of Cr and Mo in case of Mo-containing alloys [1-3]. However, the specific roles of minor elements are not well understood especially with respect to precise location of Mo relative to the oxide/metal interface, nor the atomistic processes responsible for protective-oxide layer growth and breakdown. The composition, structure and thickness of the passivating oxide films are challenging to characterize considering their nanoscale dimensions and the high electric field imposed during growth in solution. Key processes that take place within the oxide and regulate passivation are controlled by defect interactions that are atomic, ionic, and electronic in nature and currently poorly understood, often needed to be studied at the resolution and detection limits of experimental methods [4]. Common electrochemical methods such as AC and DC electrochemistry are rarely connected with atomic scale ex situ techniques such as scanning tunneling (STM), atomic force (AFM) and transmission electron (TEM) microscopies to yield crucial information on structure, molecular and electronic properties of the oxides [5]. The main goal of this work is to integrate single crystal electrochemical measurements with nanoscale characterization to advance fundamental understanding and eventually predict the roles of Cr and Mo on surface stability in corrosive environments. Such a connection between global electrochemistry and atomic studies is infrequently reported [6].Polycrystalline alloys contain many grain orientations that may affect passivation behavior [7]. This work focuses on examining the oxide growth kinetics on single crystals. It is expected that grain surface facet evolution and oxide thickness will vary according to crystallographic orientation [8, 9]. Model alloys Ni-22Cr and Ni-22Cr-6Mo, wt%, were arc-melted, cold-rolled to achieve ~12% deformation, and then heat treated to allow for re-crystallization and growth of sufficiently large grains to enable electrochemical measurements. A ~4 mm2 area of a sample was first scanned with Electron Backscattered Diffraction (EBSD) to obtain a “catalog” of grain orientations, and then each grain within the EBSD map was probed regarding its electrochemical behavior. The kinetics of the passive state of single grains were determined potentiostatically at +0.2 VSCE in solution, where a previously formed on air oxide was partially reduced at -1.3 VSCE. Electrochemical impedance spectroscopy (EIS) was then collected after various stages of oxide growth and the data was fit assuming a constant phase element to enable calculation of thickness [10]. Oxide charge formation, current density, and thickness for each alloy and grain orientation were studied using Mott-Cabrera and Point Defect models. Ex situ Raman and XPS conducted on polycrystalline samples aided in determining molecular identity and the efficiency of oxide growth versus dissolution. AFM topography images and profiles across single grain surfaces at intermediate stages of potentiostatically controlled oxide growth were obtained to give insight on the nucleation and morphology of oxides formed on various grains (Figure 1a). Furthermore, the structural and electrochemical properties of oxides grown potentiostatically on specific crystallographic orientations were correlated with the TEM analysis performed on the oxides grown chemically under equivalent conditions using potassium persulfate or hydrogen peroxide oxidants. It was found the EIS, TEM and XPS oxide thicknesses were in agreement with each other. The broader relevancy of this presentation, besides connecting electrochemistry and nano- to mesoscale characterization, is its contribution to concepts beneficial towards development of predictive correlations associated with the passivity of Ni alloys during aqueous exposure. Acknowledgements This project was funded by U.S. ONR (SP0028970-PROJ0007990) and MURI Program under the direction of Dr David Shifler (N00014-14-1-0675).

Conductive AFM Topography of Intrinsic Conductivity Variations in Silica Based Dielectrics for Memory Applications

As flash memory approaches its scaling limit, resistive RAM devices show promise for next generation memories due to their small size and low operating voltages enabling denser packing and lower power consumption. Many materials have shown desirable active layer properties, yet silicon based memories are particularly appealing as they allow for facile integration into CMOS infrastructures. Our devices are composed of silicon suboxide sandwiched between conductive electrodes. Here, we present results probing the three-dimensional structure of conducting pathways following the application of an electric field. These pathways, or filaments, give dielectric materials memory functionality yet are difficult to image with conventional techniques owing to low imaging contrast. Our results highlight the importance of conductive atomic force microscopy for three-dimensional analyses and show that filamentation conforms to the pristine dielectric structure. We also present findings showing that multiple growths can compete during the filament growth process.

3D-Interconnected Channels of Ordered Mesoporous Carbon (3D-OMC) : Effective Electrode Material for Electrochemical Supercapacitors

Alarming of global climate change by increasing strategy of atmospheric CO2 emissions and dwindling of fossil fuel leads to energy associated queries in near future. For complementary, generation/utilization of alternate energy is an important promise. Different type of energies (conventional/non-conventional) were developed that can meet the present day power demands. Batteries, fuel cells and supercapacitors are the best candidates for storing/conversion of the generated energy.1 In modern days supercapacitors have much more attention than others because of its high power density, slow discharging capacity with ultra fast charging and good cycling stability with high capacitance retention. In addition light weight, flexibility and effective performance in energy storage it is focused for future in backup power systems, electric vehicles and electronic components. The electric double layer formed at the interface between an electronically conductive material and an electrolyte solution is the basic mechanism for charge storage performance.1,2 Carbon and its allotropes like CNT, graphene etc., plays a major role in supercapacitors fabrication due to their unique structural morphology, excellent conductivity and electrochemical stability. The controlled morphology, high surface area with porous nature may increase the easy accessibility of electrolyte.3,4Herein, we report the 3D-interconnected channels of ordered mesoporous carbon (3D-OMC) as an electrode material for electrochemical supercapacitors with enhanced performance over with Vulcan carbon. In the present study, 3D-OMC was synthesized by using mesoporous silica KIT-6 as template and sucrose was used as carbon source. The as synthesized material was characterized with Low angle XRD (crystanallity), Laser Raman (microstructure), BET (surface area), SEM & TEM (Morphology) and AFM (topography). From all the above, the 3D-OMC has IG/IDratio of 1.06, surface area, pore volume and average pore size as 750 m²/g, 0.88 cm³/g and 4.6 nm respectively with well arranged porous network of 3D-carbon. Electrochemical studies of the synthesized material was investigated by cyclic voltammetry (CV) and Galvanostatic Charge-Discharge cycle test in acidic medium and made comparison over with Vulcan carbon (VC). The existence of typical rectangular-shaped voltammogram reveals that an ideal electrical double layer capacitor (EDLC) behavior arisen from the charge accumulation at electrode/ electrolyte interface. Eventhough both materials shows the EDLC, the integrated area of the rectangular shape of 3D-OMC electrode was high compared to VC due to the pours nature and the ability towards interaction between electrolyte. On account of the encircled area of CV, current density increases with a gradual increase of scan rate (5 to 300 mV) reveals a superior specific capacitance as a result of major involvement of EDLC mechanism. In addition a couple of wide and vague peaks were observed from the faradic redox reactions (pseudo-capacitance) that performed due to the involvement of oxygen-containing groups on the carbon surface.3,5 The discharge (linear voltage versus time) profile of 3D-OMC shows a superior symmetrical charge/discharge performance than VC. Specific capacitance of the 3D-OMC (206 F/g) is higher when compared with VC (58 F/g) at 0.5 A/g and also shows a good cycling stability with high retention. Hence the enhanced specific capacitance of 3D-OMC is considered one of the most promising materials for electric double layer capacitors (EDLC) for future energy storage applications.

Mismatch detection in DNA monolayers by atomic force microscopy and electrochemical impedance spectroscopy.

Background: DNA hybridization is at the basis of most current technologies for genotyping and sequencing, due to the unique properties of DNA base-pairing that guarantee a high grade of selectivity. Nonetheless the presence of single base mismatches or not perfectly matched sequences can affect the response of the devices and the major challenge is, nowadays, to distinguish a mismatch of a single base and, at the same time, unequivocally differentiate devices read-out of fully and partially matching sequences.Results: We present here two platforms based on different sensing strategies, to detect mismatched and/or perfectly matched complementary DNA strands hybridization into ssDNA oligonucleotide monolayers. The first platform exploits atomic force microscopy-based nanolithography to create ssDNA nano-arrays on gold surfaces. AFM topography measurements then monitor the variation of height of the nanostructures upon biorecognition and then follow annealing at different temperatures. This strategy allowed us to clearly detect the presence of mismatches. The second strategy exploits the change in capacitance at the interface between an ssDNA-functionalized gold electrode and the solution due to the hybridization process in a miniaturized electrochemical cell. Through electrochemical impedance spectroscopy measurements on extended ssDNA self-assembled monolayers we followed in real-time the variation of capacitance, being able to distinguish, through the difference in hybridization kinetics, not only the presence of single, double or triple mismatches in the complementary sequence, but also the position of the mismatched base pair with respect to the electrode surface.Conclusion: We demonstrate here two platforms based on different sensing strategies as sensitive and selective tools to discriminate mismatches. Our assays are ready for parallelization and can be used in the detection and quantification of single nucleotide mismatches in microRNAs or in genomic DNA.

Evaluation of Polyesterimide Nanocomposites Using Methods of Thermal Analysis

Polyesterimide resin applied for winding impregnation has been modified by incorporating the hydrophilic and hydrophobic nanosilica, montmorillonite and aluminium oxide. For assessment of the resins in liquid and cured states thermoanalytical methods TG/DSC were used. For pure and nanofilled resins the results of investigation of AFM topography, bond strength, dielectric strength and partial discharge resistance have been also presented. It was found that dielectric and mechanical properties of polyesterimide resin containing hydrophilic silica as well aluminium oxide were much improved as compared to pure resin. Based on our investigations we have found that the methods of thermal analysis may be very useful for evaluation of nanocomposites: DSC/TGA study of resins in the liquid state under dynamic conditions can be applied to pre-select nanocomposites; isothermal TG curves of cured resins can be utilized for thermal stability evaluation; in turn, TG study after thermal ageing of cured resins could confirm the barrier properties of nanocomposites.

Protein clustering in chemically stressed HeLa cells studied by infrared nanospectroscopy.

Photo-Thermal Induced Resonance (PTIR) nanospectroscopy, tuned towards amide-I absorption, was used to study the distribution of proteic material in 34 different HeLa cells, of which 18 were chemically stressed by oxidative stress with Na3AsO3. The cell nucleus was found to provide a weaker amide-I signal than the surrounding cytoplasm, while the strongest PTIR signal comes from the perinuclear region. AFM topography shows that the cells exposed to oxidative stress undergo a volume reduction with respect to the control cells, through an accumulation of the proteic material around and above the nucleus. This is confirmed by the PTIR maps of the cytoplasm, where the pixels providing a high amide-I signal were identified with a space resolution of ∼300 × 300 nm. By analyzing their distribution with two different statistical procedures we found that the probability to find protein clusters smaller than 0.6 μm in the cytoplasm of stressed HeLa cells is higher by 35% than in the control cells. These results indicate that it is possible to study proteic clustering within single cells by label-free optical nanospectroscopy.

Caracterización mecánica de recubrimientos de aluminio por CVD-FBR sobre aceros inoxidables y resistencia a la oxidación en vapor de agua

<p>Los recubrimientos de aluminio depositados sobre el acero inoxidable austenítico AISI 317 por Deposición Química de Vapor en Lecho Fluidizado (CVD-FBR) presentan a altas temperaturas una reducción de la velocidad de corrosión de más de 80 veces. Se realizó la caracterización mecánica de los recubrimientos por medio de microdureza, nanoindentación, para conocer cómo se vieron afectas las propiedades mecánicas (en especial la dureza y el módulo de Young) del recubrimiento y del sustrato luego de ser sometidos a la oxidación a alta temperatura. También se hicieron análisis por medio de Microscopia Electrónica de Barrido (MEB), para observar los cambios microestructurales, y de Microscopia de Fuerza Atómica (MFA), para observar cómo varía la topografía y el gradiente de rugosidad en función de la distancia recorrida por la punta del cantiléver durante los barridos.</p>

Tip induced mechanical deformation of epitaxial graphene grown on reconstructed 6H–SiC(0001) surface during scanning tunneling and atomic force microscopy studies

The structural and mechanical properties of an epitaxial graphene (EG) monolayer thermally grown on top of a 6H–SiC(0001) surface were studied by combined dynamic scanning tunneling microscopy (STM) and frequency modulation atomic force microscopy (FM-AFM). Experimental STM, dynamic STM and AFM images of EG on 6H–SiC(0001) show a lattice with a 1.9 nm period corresponding to the (6 × 6) quasi-cell of the SiC surface. The corrugation amplitude of this (6 × 6) quasi-cell, measured from AFM topographies, increases with the setpoint value of the frequency shift Δf (15–20 Hz, repulsive interaction). Excitation variations map obtained simultaneously with the AFM topography shows that larger dissipation values are measured in between the topographical bumps of the (6 × 6) quasi-cell. These results demonstrate that the AFM tip deforms the graphene monolayer. During recording in dynamic STM mode, a frequency shift (Δf) map is obtained in which Δf values range from 41 to 47 Hz (repulsive interaction). As a result, we deduced that the STM tip, also, provokes local mechanical distortions of the graphene monolayer. The origin of these tip-induced distortions is discussed in terms of electronic and mechanical properties of EG on 6H–SiC(0001).

Very low roughness MAPLE-deposited films of a light emitting polymer: an alternative to spin coating

The matrix assisted pulsed laser evaporation (MAPLE) technique is emerging as an alternative route to conventional deposition methods of organic materials (solution-phase and thermal evaporation approaches). However, the high surface roughness of the films deposited by MAPLE makes this technique not compatible with applications in electronics and photonics. In this paper we report the deposition of MAPLE-films of a green light emitting polymer, commercially named ADS125GE, with remarkable low roughness values, down to about 10 nm at the thickness conventionally used in photonic devices (~40 nm). This issue is discussed as a function of polymer concentration, target-substrate distance and substrate rotation based on AFM topography images, roughness estimation and optical (absorption and luminescent) measurements. In addition we have fabricated an organic light emitting diode with this technique using the best deposition parameters which guarantee the lowest roughness. These results open the way to MAPLE applications in organic photonics and opto-electronics.

Structure Influence on Charge Transport in Naphthalenediimide–Thiophene Copolymers

Reported here is a characterization of a series of NDI–thiophene copolymers with one, two, three, and four thiophene units synthesized using Stille polycondensation of dibromo-naphthalene diimide and the trimethylstannylthiophene monomers. The effect of extension of the thiophene donor group is studied in terms of structure-charge transport correlation. The influence of side chains located on the thiophene units of copolymers with two and four thiophene units per monomer is also investigated. Charge transport of both signs is studied experimentally in field-effect transistors. Microstructural data obtained by near-edge X-ray absorption fine structure (NEXAFS) and grazing incidence wide-angle X-ray scattering (GIWAXS) is supported by AFM topography scans. Ultraviolet photoelectron spectroscopy (UPS) and UV–vis spectroscopy data are employed in the measurement of energy levels, and changes with annealing temperature are also discussed. Most of the polymers reach excellent electron and hole mobility with one copolymer (NDI-T4) exhibiting an especially balanced ambipolar charge transport of 0.03 cm2 V–1 s–1. An odd–even effect in hole mobility is observed with higher values for polymers with an even number of thiophene units. The reported findings indicate that the final charge transport properties are a result of the interplay of many factors, including crystallinity, planarity and linearity of chain, spacing between acceptor units and packing of solubilizing branched side chains.

Optical properties and electrical transport of thin films of terbium(III) bis(phthalocyanine) on cobalt.

The optical and electrical properties of terbium(III) bis(phthalocyanine) (TbPc2) films on cobalt substrates were studied using variable angle spectroscopic ellipsometry (VASE) and current sensing atomic force microscopy (cs-AFM). Thin films of TbPc2 with a thickness between 18 nm and 87 nm were prepared by organic molecular beam deposition onto a cobalt layer grown by electron beam evaporation. The molecular orientation of the molecules on the metallic film was estimated from the analysis of the spectroscopic ellipsometry data. A detailed analysis of the AFM topography shows that the TbPc2 films consist of islands which increase in size with the thickness of the organic film. Furthermore, the cs-AFM technique allows local variations of the organic film topography to be correlated with electrical transport properties. Local current mapping as well as local I–V spectroscopy shows that despite the granular structure of the films, the electrical transport is uniform through the organic films on the microscale. The AFM-based electrical measurements allow the local charge carrier mobility of the TbPc2 thin films to be quantified with nanoscale resolution.

Influence of Bound Metals on the Electrical Properties of Single Molecule Junction Porphyrin-Imides Linked to SWNTs

Since Aviram and Ratner proposed that a single molecule could act as a molecular rectifier, containing a donor and an acceptor separated by a sigma bond, many such molecules have been synthesized for use in devices. Among these, porphyrins are widely used as molecular electronic devices owing to their structural flexibility as well as their well-developed synthetic chemistry, which allows their physical and chemical properties to be adjusted, making them ideally suited for binding to metals. In this research, porphyrin-imide dyads with perpendicular arrangements containing different metals (zinc and rhodium) bound to porphyrin molecules have been synthesized (Figure 1) and the effect of these “center metals” on the electrical properties of single molecule junction porphyrin-imides connected to SWNTs have been investigated. DFT calculations for porphyrin-imide dyads revealed that the HOMO and LUMO energy levels of the dyads were completely separated, indicating the absence of any intramolecular interactions between the donor and acceptor. Furthermore, these calculations also revealed that the molecular orbital of rhodium porphyrin-imide for HOMO-2, HOMO-3, and LUMO-1 were entirely localized on the rhodium center, whereas no localization on the zinc center of the molecular orbital for zinc porphyrin-imide was observed. Based on these DFT results, it was predicted that employing zinc and rhodium as center metals would result in the corresponding dyads having different electrical properties. The connection of single molecule junctions for porphyrin-imides linked covalently to SWNTs was evidenced by the coordination of the rhodium porphyrin-imide dyad with pyridine-tert-dodec-AuNPs (~3 nm in size) used as a marker. AFM topography was employed in order to confirm that the AuNPs were attached to the molecule. Moreover, the topography revealed the presence of particles with a height of 2.6 nm, which subsequent TEM observations revealed to be part of a single-molecule junction (Figure 2c). The electrical properties of these dyads were measured by point-contact current imaging atomic force microscopy (PCI-AFM). The results for the rectification behavior of single molecule junction zinc porphyrin-imide revealed larger values, with an average rectification ratio of 26, compared to the rhodium porphyrin-imide that had an average rectification ratio of 6. This differential conductance indicated that the HOMO-LUMO gap can be controlled by changing the center metal.

Erosion of Electrochemically Generated Metastable Metal Surfaces (MMS) on Gold and Platinum Electrodes

Erosion of Electrochemically Generated Metastable Metal Surfaces (MMS) on Gold and Platinum Electrodes