Atomic Force Miscroscopy Research Articles

Ultrathin Bi4O5Br2 nanosheets for selective photocatalytic CO2 conversion into CO

The ultrathin two-dimensional materials with the unique catalytic performance attract extensive interests of researchers. In this paper, ultrathin Bi4O5Br2 nanosheet (Bi4O5Br2-UN) is synthesized through precursor method by controlling the mole ratio of Bi(NO3)3·5H2O and cetyltrimethyl ammonium bromide. The ultrathin structure of Bi4O5Br2-UN was confirmed by X-ray diffraction, atomic force miscroscopy and high-resolution transmission electron microscopy. UV–vis light irradiation, Bi4O5Br2-UN display excellent selective photocatalytic activity for CO2 conversion, which also was proved by isotope experiment. After 2 h reaction, the CO generation over Bi4O5Br2-UN reached 63.13 μmol g−1, which is about 2.3 times higher than that of bulk Bi4O5Br2 (27.56 μmol g−1). Additionally, the CO selectivity of Bi4O5Br2-UN is more than 99.5% in the whole catalytic process. The enhanced CO2 adsorption, suitable band position and improved charge separation induce the outstanding photocatalytic performance of Bi4O5Br2-UN. The photocatalytic mechanism of CO2 conversion with Bi4O5Br2-UN was analyzed by the in situ Fourier transform infrared spectrometry (FT-IR).

Local Photovoltaic Properties of Graphene–Silicon Heterojunctions

Interfacing of materials with different dimensionalities becomes increasingly relevant for many applications which can utilize the exceptional properties of low‐dimensional materials on one hand, and build‐up on the existing production know‐how for bulk (3D) materials on the other. Numerous appealing possibilities are offered by such combinations. In this work, the authors focus on 2D–3D heterostructures composed of mechanically exfoliated single‐ and few‐layer layer graphene (Gr) coupled to freshly etched n‐doped silicon. Two ways of characterizing the photovoltaic (PV) properties of such junctions by Conductive Atomic Force Miscroscopy (C‐AFM) under illumination are shown: 1) by measuring the I–V curves directly at selected points, while the flakes are scanned in intermittent mode and 2) by scanning the samples in contact mode at different bias voltages, followed by reconstruction of I–V curves using mean photocurrent quantified in selected areas. The direct I–V curves measurement has been employed to discriminate the effect of increased p‐doping of the graphene layer, and the contact mode has been utilized to evaluate the separation between the graphene flake and the substrate. Additionally, pros and cons of the two routes are briefly discussed and outlook for further advanced nanoscale characterization of such junctions is proposed.

TL and OSL studies on undoped diamond films grown by hot filament chemical vapor deposition

In this work, approximately 0.5µm thick diamond films were grown on a silicon substrate by hot filament chemical vapour deposition (HFCVD) method in a gas mixture of hydrogen and methane. The batch to batch reproducibility of the sample using this technique was found to be very good. The obtained film was characterized by micro laser Raman spectroscopy (MLRS), grazing incidence X-ray diffractometry (GIXRD), scanning electron microscopy (SEM) and atomic force miscroscopy (AFM) techniques. MLRS and GIXRD results confirmed the formation of diamond whereas SEM and AFM analyses indicated uniform morphology of the film with an average grain size of 200nm. The deposited film was studied for ionizing radiation dosimetry applications using the thermoluminescence (TL) and optically stimulated luminescence (OSL) techniques after irradiating the film by a calibrated 5mCi, 90Sr/90Y beta source. In the TL measurement, for a heating rate of 4K/s, broad glow curve was obtained which was deconvoluted into seven TL peaks. The integrated TL counts were found to vary linearly with increasing the radiation dose up to 10kGy. The characteristic TL output seen in the temperature range 200–300°C, may be considered good for thermal stability of the film and it could also avoid TL fading during storage and non-interference of any black body radiation during the measurement. However, in comparison to TL output, the OSL response for 470nm LED stimulation was found to be lesser. The CW–OSL decay curve has shown two components contributing to the OSL signal, having photoionization cross-section 1.5×10−18 and 5.2×10−19cm2 respectively. The studies have revealed the possibility of using diamond film for high dose radiation dosimetry with TL/OSL method.

A photochemical proposal for the preparation of ZnAl2O4 and MgAl2O4 thin films from β-diketonate complex precursors

ZnAl2O4 and MgAl2O4 thin films were grown on Si(100) and quartz plate substrates using a photochemical method in the solid phase with thin films of β-diketonate complexes as the precursors. The films were deposited by spin-coating and subsequently photolyzed at room temperature using 254nm UV light. The photolysis of these films results in the deposition of metal oxide thin films and fragmentation of the ligands from the coordination sphere of the complexes. The obtained samples were post-annealed at different temperatures (350–1100°C) for 2h and characterized by FT-Infrared spectroscopy, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), atomic force miscroscopy (AFM), and UV–vis spectroscopy. The results indicate the formation of spinel-type structures and other phases. These characteristics determined the quality of the films, which were obtained from the photodeposition of ternary metal oxides.

AFM Scratching for Adhesion Studies of Thin Polymer Coatings on Steel

Atomic force miscroscopy (AFM) scratching at constant applied forces was used to quantify the adhesion of polymer coatings to cold rolled steel (CRS) and to study the effectiveness of a pretreatment for improving the adhesion. The pretreatment was a phosphate-free zirconia-based coating. Thin layers of commercially available epoxy, acrylic and polyester-based polymer coatings, were applied to polished or pretreated cold rolled steel substrates and the surface was scratched at the edge of the polymer coatings with the AFM tip at increasing values of normal loads until the coating was removed. Adhesion strengths were determined from the minimum tip-sample interaction force and number of cycles (scans) at a particular applied force. The pretreatment significantly improved adhesion of the epoxy and acrylic-based coatings on CRS. Adhesion of the acrylic-based coating was found to be better than the epoxy coatings on the bare as well as pretreated steel. Adhesive strength of the polyester-based coating was inconclusive because it was very easily removed on application of small forces using the AFM tip. The AFM scratching technique was found to provide a quick, easy and effective way to make quantifiable comparisons in relative adhesive strengths of polymer coatings and the effect of pretreatments.

High concentration few-layer graphene sheets obtained by liquid phase exfoliation of graphite in ionic liquid

In the present work, the use of a commercial ionic liquid as a convenient solvent medium for graphite exfoliation in mild and easy conditions without any chemical modification is presented. To confirm the presence of few layer graphene, its dispersion was charachterized by Raman spectroscopy, atomic force miscroscopy and field emission electron microscopy. From the UV-Vis characterization, a graphene concentration as high as 2.543 mg/ml was obtained, which is the highest value reported so far in any solvent.

Conductive Properties of Switchable Photoluminescence Thermosetting Systems Based on Liquid Crystals

Conductive properties of different thermosetting materials modified with nematic 4'-(hexyl)-4-biphenyl-carbonitrile (HBC) liquid crystal and rutile TiO(2) nanoparticles were successfully studied by means of tunneling atomic force miscroscopy (TUNA). Taking into account the liquid crystal state of the HBC at room temperature, depending on both the HBC content and the presence of TiO(2) nanoparticles, designed materials showed different TUNA currents passed through the sample. The addition of TiO(2) nanoparticles into the systems multiply the detected current if compared to the thermosetting systems without TiO(2) nanoparticles and simultaneously stabilized the current passed through the sample, making the process reversible since the absolute current values were almost the same applying both negative and positive voltage. Moreover, thermosetting systems modified with liquid crystals with and without TiO(2) nanoparticles are photoluminescence switchable materials as a function of temperature gradient during repeatable heating/cooling cycle. Conductive properties of switchable photoluminescence thermosetting systems based on liquid crystals can allow them to find potential application in the field of photoresponsive devices, with a high contrast ratio between transparent and opaque states.

Tip-enhanced single molecule fluorescence near-field microscopy in aqueous environment

For nanobiophysical applications, scanning near-field optical microscopy must combine high optical resolution and single fluorescent molecule sensitivity with the ability to operate in aqueous solution. These requirements can be achieved using the electric field enhancement at the tip of illuminated silicon probes for atomic force miscroscopy (AFM), whereby single ATTO-740 dye molecules could be imaged at an optical resolution down to 20 nm under ambient conditions as well as in aqueous solution. Two illumination modes have been tested: (a) bottom illumination in a total internal reflection microscopy setup and (b) direct top illumination, both with dedicated phase-sensitive single photon counting technology in dynamic AFM mode of operation.

AFM Study of the Stability of a Dense Affinity-Bound Liposome Layer

Liposomes that are surface-bound to a biomaterial such as a contact lens are of interest for localized delivery of therapeutic agents, but it is not known whether such liposome layers are sufficiently robust. The stability of a dense, PEG-functionalized layer of liposomes, affinity-bound onto a multilayer coated surface, was tested under various stress conditions using colloid-probe atomic force miscroscopy (AFM). The different stress effects were generated by varying the applied normal load of the probe and the impinging fluid shear through different approach velocities and by varying the applied lateral forces by scanning under increasing force loads. The effect of applied forces (normal and lateral) was further investigated by coating the probe with a layer of albumin. The liposomes remained intact following the ramping of both protein-coated and uncoated probes under the normal and lateral loads. The low-fouling nature of these liposomes, with respect to nonspecific protein adsorption, was also demonstrated from the interaction force measurements which showed only weak adhesion from the protein layer during the contact period of the albumin-coated probe. The observed adhesive interactions were concluded to be a direct result of the applied load from the probe, during the force measurements, rather than from attraction of the protein molecules for the surface-bound liposomes. The low frictional response of the liposome layer indicated the viscoelastic nature of these molecules, which enabled liposome structure retention during the continuous load application. The demonstrated stability of the liposomes presents a system of viable and localized drug delivery in, for example, ophthalmic applications.

Characterization of commercial water treatment membranes modified via ion beam irradiation

A commercial sulfonated polysulfone water treatment membrane was modified by ion beam irradiation. After irradiation, attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) was used to identify chemical structure changes incurred by the modification. Some of the sulphonic groups on the surface of the membrane were broken due to ion beam irradiation, which resulted in crosslinking of the polymer. These changes modified the surface morphology of the membrane and also decreased the negative charge of the membrane. Previous studies have determined that ion beam irradiation produces no significant changes in membrane permeability along with improvements in selectivity with respect of organic carbon rejection. Thus, this study focused on conducting bench-scale cross-flow filtration experiments to investigate the cake accumulation on the modified membrane. Through ATR-FTIR and atomic force miscroscopy, it was concluded that fouling of the modified membrane was significantly less severe than that of the virgin membrane. Thus, ion beam irradiation was observed to be an effective membrane modification technique.

Characteristics of low-temperature-grown GaN films on Si(111)

In this paper, we report on the characteristics of GaN films grown on Si(111) at a low temperature (200 °C) by electron cyclotron resonance (ECR) plasma-assisted metalorganic chemical vapor deposition (PA-MOCVD). Structural analysis of the GaN films was performed by using scanning electron microscopy (SEM), atomic force miscroscopy (AFM), X-ray diffraction (XRD), energy dispersive X-ray analysis (EDX), and Rutherford backscattering spectrometry (RBS) . Post deposition analysis revealed high quality crystalline GaN was obtained at this low temperature. Electrical analysis of the GaN films was done by using current–voltage ( I– V) measurements where electrical characterizations were carried on GaN/Si heterojunction and Schottky barrier diodes. Rectification behaviour was observed for the isotype GaN/Si (n–n) heterojunction. Ideality factors and Schottky barrier heights for Ni and Cr Schottky barriers on GaN, were deduced to be 1.4 and 1.7; and 0.62 and 0.64 eV, respectively.

Observation of Twisting Growth of Branched Polyethylene Single Crystals Formed from the Melt

The twisting growth of a branched polyethylene single crystal formed from the melt was observed directly by means of transmission electron and atomic force miscroscopy. The surface stress asymmetry arising from the asymmetry of the surface-fold structure and, chain tilting resulted in the twisting growth of the single crystals. The handedness of the twisting lamellae was consistent With the chain-tilting direction. When multilayer lamellae piled up in a thicker film, the lamellar twist would be inevitably causing screw dislocations.