Search

Atomic force microscopy (AFM) is one of the most important tools in nanotechnology and surface science. Because of recent developments, nowadays, it is also used to study dynamic processes, such as thin film growth and surface reaction mechanisms. These processes often take place at high temperature and there is a clear need to extend the current operating temperature range of AFM. This letter describes a heating stage and a modified AFM that extends the maximum operating temperature to 750 °C. Atomic step resolution is obtained up to 500 °C in ambient and even up to 750 °C in vacuum. [ABSTRACT FROM AUTHOR]

Epitaxially grown W/NbN nanolayers are superlattice materials that exhibit a hardness at small bilayer repeat periods which exceed the hardness predicted by the rule of mixtures for normal composites. Interfaces in the bilayered superlattice play a critical role in the superhardening process. The objective of this investigation was to examine the behavior of the superlattice material, W/NbN. Nanoindentations and in situ surface imaging were conducted over a range of applied loads on samples of W/NbN with two different bilayer periods (Λ=5.6 and 10.4 nm), and monolithic samples of the niobium nitride (NbN) ceramic and the tungsten (W) metal which comprise the superlattice material. Shallow nanoindentations were made to a depth equal to the individual layer thicknesses of the epitaxially grown W/NbN nanolayers in order to investigate the individual interface effect. The mechanical properties were determined using the Oliver and Pharr method and compared for all the samples. The energies of indentation are calculated. The characteristics of the material pileup resulting from the nanoindentations are determined from the scanned surface images. An increase in hardness is observed in the superlattice materials at deeper indentation depths. The results indicate that this increase in hardness is related to the nature of the interface between the layers in the superlattice materials. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We report the implementation of a dilution refrigerator-based scanning microwave impedance microscope with a base temperature of ∼100 mK. The vibration noise of our apparatus with tuning-fork feedback control is as low as 1 nm. Using this setup, we have demonstrated the imaging of quantum anomalous Hall states in magnetically (Cr and V) doped (Bi, Sb)2Te3 thin films grown on mica substrates. Both the conductive edge modes and topological phase transitions near the coercive fields of Cr- and V-doped layers are visualized in the field-dependent results. Our study establishes the experimental platform for investigating nanoscale quantum phenomena at ultralow temperatures. [ABSTRACT FROM AUTHOR]

A clear description of how surface morphology is affected by the bonding of biomolecules with metal surfaces is critical to identify due to the potential applications in microelectronics, medical devices, and biosensors. Amino acids (AAs) on bare Au(111) were previously observed to trap Au adatoms, eventually leading to the formation of one atom high metal islands. To better understand the role of surface identity, L-isoleucine on Au(111) modified with a Ag thin film was investigated at ambient conditions with electrochemical scanning tunneling microscopy. The mere presence of an Ag monolayer drastically changed the amino acid/surface interactions despite the chemical similarity of Au and Ag. The adsorption of the AAs on the Ag monolayer drastically altered the surface and caused significant surface roughening distinct from 2D growth which had previously existed only on top of the surface. This roughening occurred layer-by-layer and was not restricted to the first layer of the surface as seen with sulfur containing molecules. Notably, this study demonstrates surface roughening that is occurring under extremely mild conditions highlighting the ability of Ag thin films to markedly alter surface chemistry in concert with biomolecules. [ABSTRACT FROM AUTHOR]

Imaging atomically resolved surfaces and performing spectroscopy of exotic surfaces at cryogenic temperature in the presence of the magnetic field is an engineering challenge. Additionally, performing these measurements in an all-cryogen-free environment compounds the above complexity due to the associated vibration and acoustic noise generated by the running of cryogenic cold heads. We here report successful integration of a cryogen-free scanning tunneling microscope (STM) with a cryogen-free superconducting vector-magnet, connected to an ultra-high vacuum cluster assembly for in situ sample transfer. We present details of the integration involving vibration and electrical noise isolation procedures allowing for operation of the STM at extremely low noise levels below 30 fA / Hz during normal operations of the complete vacuum-line assembly with multiple turbomolecular pumps. We demonstrate the above STM capability at cryogenic temperature and in the presence of the magnetic field through atomic resolution imaging of graphite and thin films of gold on the mica substrate transferred in situ to the STM chamber. We also demonstrate spectroscopy signatures of the superconducting gap in MgB2 thin films. The design of our in-house customized cluster-vacuum-line assembly provides unsought opportunities in continuous uninterrupted imaging of ultra-clean in-vacuum grown surfaces without the need for cryogenic refills in either the STM or the magnet. [ABSTRACT FROM AUTHOR]

Practical catalysts with a porous framework, such as zeolites, host catalytic reactions at active sites engrained in the pores and channels of the scaffold. The mechanism of interaction at these active sites, defining catalyst performance, remains elusive, in large part, due to the lack of surface characterization methods available for thick films or powders. Here, we present thin film analogs of practical catalysts that allow for the implementation of surface characterization tools, including advanced microscopy and operando spectroscopy methodologies. Specifically, we investigated bilayer silica, MFI nanosheets, and UiO-66 thin films using a multi-modal approach addressing film growth, characterization, and gas adsorption aimed at understanding catalytic activity, reactivity, and selectivity properties, as defined by molecular-level changes in the reaction mechanism. [ABSTRACT FROM AUTHOR]

An often-overlooked property of transition metal oxide thin films is their microscopic surface structure and its effect on the electronic properties in the ultrathin limit. Contrary to the expected conservation of the perovskite stacking order in the (001) direction, heteroepitaxially grown SrIrO3 films on TiO2-terminated SrTiO3 are found to exhibit a terminating SrO surface layer. The proposed mechanism for the self-organized conversion involves the adsorption of excess oxygen ions at the apical sites of the IrO2-terminated surface and the subsequent decomposition of the IrO6 octahedra into gaseous molecular IrO3 and the remaining SrO-terminated surface. Whereas the ab initio calculated electronic structure of SrO-terminated SrIrO3 in the monolayer limit exhibits a striking similarity to bulk Sr 2 IrO 4 , the broken octahedral symmetry at the IrO2-terminated surface would mix the otherwise crystal field split eg and t 2 g states, resulting in distinctly different low-energy electronic states. [ABSTRACT FROM AUTHOR]

The delicate balance between elastic energy and electrostatic energy in highly strained BiFeO3 (BFO) thin films results in complex mixed-phase patterns, which poses significant challenges for theoretical understanding and complicates the realization of its full potential in magnetoelectric, electromechanical, and photovoltaic devices. In this letter, we explore in-plane electric field induced phase transition in strain engineered BFO thin films and elucidate the mechanism behind the assembly behavior of complex nano-scale phase domains. Our approach enables deterministic control of phase variants with well-defined structures and orientation, paving the way for designing novel data storage devices based on mixed phase BFO. [ABSTRACT FROM AUTHOR]

Zn-ZnO superhydrophobic thin films have been prepared by thermal oxidation of sputtered Zn. The superhydrophobicity observed in these coatings is attributed to multiscale roughness in the nanometric range only. The higher scale roughness was due to the combination of nanoclusters (solid regions) and air gaps while the lower scale roughness was due to the textured surface created by the fusion of individual ZnO nanocrystals to form the nanoclusters. The superhydrophobicity in these coatings has been observed only for an optimum combination of solid regions (i.e., nanoclusters) and air pockets. Experimental evidences have been provided to demonstrate that an additional micron-scale roughness on the substrate does not affect the wettability of the coating. It has been observed that the wettability of the Zn-ZnO coatings changes from hydrophobic to superhydrophobic during the initial 24 h after deposition. This occurs due to the outward self-diffusion of Zn to the surface and its subsequent oxidation under ambient conditions. Field-emission scanning electron microscopy (FESEM) and atomic force microscopy (AFM) were used to demonstrate the morphological changes while micro-Raman spectroscopy was used to record the chemical changes on the coating surface as a result of the outward diffusion and subsequent oxidation of Zn. Studies have also been carried out to determine the effect of UV irradiation on the Zn-ZnO coatings. The UV irradiation transformed the Zn-ZnO surface from superhydrophobic to hydrophilic. Our studies based on FESEM, AFM, micro-Raman spectroscopy, and roughness profilometry show that this transformation was mainly due to the morphological changes that occur in addition to the chemical changes taking place on the ZnO surface under the influence of UV irradiation. The UV irradiation disturbs the optimum density of air pockets, leading to the loss of superhydrophobicity. [ABSTRACT FROM AUTHOR]

Generation of charged nanoparticles in the gas phase has been frequently reported in many chemical vapor deposition (CVD) processes. In an effort to confirm whether charged nanoparticles are generated during the synthesis of silicon films by CVD, a differential mobility analyzer combined with a Faraday cup electrometer was connected to an atmospheric-pressure CVD reactor under typical conditions for silicon film growth. The size distribution of positively and negatively charged nanoparticles abundantly generated in the gas phase could be measured. An electric bias applied to the substrate holder affects the growth rate, the crystallinity and the morphology of the deposited films. The bias effect indicated that charged nanoparticles were actively involved in the film growth. [ABSTRACT FROM AUTHOR]

We have studied the magnetic anisotropy of nanometric CoFe2O4 (CFO) thin films grown on (100)SrTiO3 (STO) substrates. It has been found that epitaxial substrate-induced compressive strain makes the normal-to-film axis harder than the in-plane directions. In agreement with some previous reports, the magnetization loops are found to display a characteristic shrinking at low fields. Detailed structural and microstructural analyses, together with a modeling of the magnetization loops, revealed that the microstructure of the films, namely, the coexistence of a continuous CFO and a distribution of pyramidal CFO huts emerging from the surface, are responsible for this peculiar feature. We argue that this behavior, which significantly impacts the magnetic properties, could be a general trend of spinel films grow on (001)STO substrates. [ABSTRACT FROM AUTHOR]

Spectrally selective CrxOy/Cr/Cr2O3 multilayer absorber coatings were deposited on copper (Cu) substrates using a pulsed sputtering system. The Cr targets were sputtered using asymmetric bipolar-pulsed dc generators in Ar+O2 and Ar plasmas to deposit a CrxOy (bottom layer)/Cr/Cr2O3 (top layer) coating. The compositions and thicknesses of the individual component layers have been optimized to achieve high absorptance (0.899–0.912) and low emittance (0.05–0.06). The x-ray diffraction data in thin film mode showed that the CrxOy/Cr/Cr2O3 coating consists of an amorphous phase; the Raman data of the coating, however, showed the presence of A1g and Eg modes, characteristic of Cr2O3. The x-ray photoelectron spectroscopy (XPS) data from near-surface region of the absorber suggested that the chemical state of Cr was in the form of Cr3+ and no phases of CrO2 and CrO3 were present. The experimental spectroscopic ellipsometric data have been fitted with theoretical models to derive the dispersion of the optical constants (n and k). The optical constants of the three layers indicate that the bottom two layers are the main absorber layers and the top Cr2O3 layer, which has higher oxygen content, acts as an antireflection coating. In order to study the thermal stability of the CrxOy/Cr/Cr2O3 coatings, they were subjected to heat treatment (in air and vacuum) at different temperatures and durations. The coating deposited on Cu substrates exhibited high solar selectivity (α/[variant_greek_epsilon]) of 0.895/0.06 even after heat treatment in air up to 300 °C for 2 h. At higher temperatures, the solar selectivity decreased significantly (e.g., α/[variant_greek_epsilon]=0.855/0.24 at 350 °C in air), which is attributed to oxidation of Cr crystallites, increased surface roughness, and formation of CuO. The formation of CuO and the increase in Cr3+ vacancies due to the outward diffusion of Cr at higher annealing temperatures were confirmed by XPS. In the case of vacuum annealing, for temperatures greater than 500 °C the outward diffusion of Cu was the dominating degradation mechanism. The microstructural stability of the absorber coatings heat treated in air (up to 325 °C) and vacuum (up to 600 °C) was confirmed by micro-Raman spectroscopy measurements. Studies on the accelerated aging tests indicated that the absorber coatings on Cu were stable in air up to 250 h at 250 °C with a solar selectivity of 0.898/0.11. [ABSTRACT FROM AUTHOR]

Polycrystalline BiFeO3 thin films were fabricated on (111)Pt/Ti/SiO2/Si substrates via Bi-acetate- and Fe-acetylacetonate-based chemical solution deposition and spin-coating techniques. The processing parameters were optimized in order to obtain films with high resistivity. The optical properties (refractive indices and extinction coefficients) were measured by means of ellipsometry (HeNe laser, λ=632.8 Å). Microstructure characterization was made by means of atomic force microscopy, grazing incidence x-ray diffractometry (XRD), and texture analysis. Additionally, powders prepared from a stoichiometric precursor were investigated by means of thermogravimetric and differential thermal analyses and XRD. It is demonstrated that the formation of perovskite-type BiFeO3 is accompanied by the appearance of bismuth oxide at low temperatures which then transforms into Bi36Fe2O57. For the films it was found that annealing in oxygen leads to higher indices of refraction, lower roughness, and smaller grain size. Complete crystallization of the films was achieved at a substantially lower temperature compared to that of the powders. A (100) (pseudocubic) out-of-plane preferred orientation was revealed for specimens annealed in air and oxygen. It is supposed that the crystal lattice of the thin film is close to cubic possibly due to stress development at the substrate/film interface. The electrical properties of the films were measured at room temperature by impedance analysis. The piezoelectric properties were determined using a laser vibrometer. Room temperature resistances measured at 1 kHz for metal-film-metal configurations for the specimens annealed in air and O2 were 14 Ω and 1.35 kΩ, respectively. This is explained in terms of the high sensitivity of the oxidation state (+2 or +3) of iron ions to oxygen stoichiometry in the specimens. Further electrical characterization of the specimen annealed in O2 revealed very low frequency dispersion of the dielectric constant. A dielectric loss of 1% or less was detected in a wide range of frequency. The films annealed in oxygen showed piezoelectric activity with a value of the piezoelectric coefficient d33 of 12 pm/V. A relatively weak ferroelectricity (remnant polarization 2Pr of approximately 1 μC/cm2) was detected for the specimens annealed in oxygen. [ABSTRACT FROM AUTHOR]

Conductive rough surfaces separated by an insulating ultrathin film comprise the core of various engineering and scientific systems. Electron tunneling through the potential barrier imposed by the insulating film is the main mechanism of charge transport across these interfaces. The strong dependence of the tunnel current on the characteristics of the interface provides an in situ means of studying the electromechanical behavior of hindered interfaces in terms of the current–voltage response. Analytical relationships for the current density versus applied voltage that include the real tunneling area are used to characterize the electrical and mechanical properties of the interfaces of rough conductive surfaces coated by an insulating thin film. It is shown that the contact load, real contact area, surface topography, mechanical properties, and insulating film characteristics, such as thickness, dielectric constant, and potential barrier, can be determined from simple electrical measurements and the use of the present analytical approach. [ABSTRACT FROM AUTHOR]

We report postgrowth micro-Raman stress imaging of cracks in (111) homoepitaxial diamond films. Undoped and phosphorous-doped diamond thin films grown by microwave plasma-enhanced chemical-vapor deposition on Ib (111)-oriented diamond substrates have been studied by confocal micro-Raman spectroscopy. For comparison purposes, a film grown on a (100) Ib substrate was also examined. Thanks to the confocal optics, the Raman signal arising from the epilayer could be discriminated from that arising from the substrate. As was already reported, the (111) films exhibited substantial tensile stress, indicated by a downshift in the Raman peak and by spontaneous cracking in films grown thicker than 5 μm. The sixfold symmetry of the cracks supported that the films were homoepitaxial. A high compressive stress was also detected at the substrate near surface, and a partial stress relaxation was observed to occur in the vicinity of the cracks. Possible origins of the high tensile stress observed in the (111) homoepitaxial diamond films are discussed. [ABSTRACT FROM AUTHOR]

Examines GaN films grown on 6H-SiC and Al2O3 substrates using low-pressure chemical vapor deposition with GaCl3 and NH3 as precursors. Optical, scanning tunneling and transmission electron microscopy results; Growth conditions; Determination of the overall shape and average diameter of microislands.

The surface topography and structure of low-pressure chemical vapor deposited silicon films on thermal oxide grown on (100) silicon substrates have been investigated after different processing steps. Atomic force microscopy topographic measurements are performed on undoped as-grown samples and after ex situ phosphorous doping from a POCl[sub 3] source. In addition, topographies of the doped films are obtained using constant current scanning tunneling microscopy on hydrofluoric acid passivated surfaces under high vacuum. As a result we have found that surface topography and roughness are mainly determined by the deposition process. Roughness of films deposited at 620 °C is related to the grain structure represented by hillocks with typical lateral dimensions between 50 and 150 nm in the images. Doping by high temperamre diffusion and subsequent annealing causes a complete recrystallisation of the film, leading to typical lateral grain sizes between 200 and 600 nm. However, the surface topography of the doped films still remains determined by the hillocks formed by the deposition process. Values of the surface roughness are between 7 and 14 nm depending on the process step. © 1997 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Presents information on a study that investigated the influence of silver doping on the transport properties of Y[sub1]Ba[sub2]Cu[sub3]O[sub7 x] thin films prepared by BaF[sub2] and copper co-evaporation and optimized ex situ post annealing. Experimental procedure; Results and discussion on the study; Conclusions.

The effect of sputtering pressure on the surface/interface microstructure, crystal phase, mechanical properties and electrical characteristics of nanocrystalline W-Mo films is reported. The W-Mo films (≈300 nm) with variable microstructure were deposited under variable argon (Ar) sputtering pressure (PAr), which is varied in the range of 3-19 mTorr. X-ray diffraction analyses indicate that the W-Mo films crystallize in thermodynamically stable α-phase of W. However, the crystal-quality degradation occurs for W-Mo films deposited at higher PAr due to difference in the adatom mobilities. The average grain size (d) of the W-Mo films was in the range of 11-24 nm; grain size decreases with increasing PAr. The effect of PAr and associated microstructure are significant on the mechanical characteristics; the hardness (H) and modulus of elasticity (Er) of W-Mo films deposited at lower PAr were higher but decreases continuously with increasing PAr. The W-Mo films deposited under optimum sputtering pressure exhibit superior mechanical characteristics: H=40 GPa, Er=275 GPa, H/Er=0.8, and H3/Er 2=0.145 GPa, which are higher compared to pure, α-phase W-films. The W-Mo films deposited at PAr=3-9 mTorr exhibit high resistivity≈350-400 μΩ-cm, which decreases to 150-200 μΩ-cm for films deposited at higher PAr. Based on the results, structure-mechanical-electrical property correlation inW-Mo films is established. [ABSTRACT FROM AUTHOR]

Inherent processes in coal gasification plants produce hazardous hydrogen sulfide (H2S), which must be continuously and efficiently detected and removed before the fuel is used for power generation. An attempt has been made in this work to fabricate tungsten oxide (WO3) thin films by radio-frequency reactive magnetron-sputter deposition. The impetus being the use of WO3 films for H2S sensors in coal gasification plants. The effect of growth temperature, which is varied in the range of 30–500 °C, on the growth and microstructure of WO3 thin films is investigated. Characterizations made using scanning electron microscopy (SEM) and x-ray diffraction (XRD) indicate that the effect of temperature is significant on the microstructure of WO3 films. XRD and SEM results indicate that the WO3 films grown at room temperature are amorphous, whereas films grown at higher temperatures are nanocrystalline. The average grain-size increases with increasing temperature. WO3 films exhibit smooth morphology at growth temperatures ≤300 °C while relatively rough at >300 °C. The analyses indicate that the nanocrystalline WO3 films grown at 100–300 °C could be the potential candidates for H2S sensor development for application in coal gasification systems. [ABSTRACT FROM AUTHOR]

A novel device for nanometer-confinement of soft matter in one dimension (1D) is presented. This nanocell, with very large (up to 106:1) cell-radius to cell-height ratio, is tailored as an accessory for quartz crystal microbalance (QCM) and QCM with dissipation-monitoring (QCM-D) sensing to study internal and interfacial energy dissipation phenomena in highly confined (in 1D) soft matter and fluid films (patent pending). The cell consists of two macroscopic plates (diameter of 9 mm), a top (the “lid”) and a bottom (the QCM-D sensor), separated by appropriate spacers with heights ranging from below 100 nm up to 10 μm. The surfaces of both the lid and the bottom plate can be mechanically or/and chemically modified, prior to cell assembly, in order to tailor desired interfacial properties for the experiment. The cell is mounted on a standard QCM-D sensor, an AT-cut quartz crystal (the quartz crystal is cut at an angle of 35° from its ZX-plane), forming the bottom plate. We illustrate theoretically and experimentally, as application examples, the use of this device for studies of dynamic mass loading and internal energy dissipation processes in thin films of ethylene glycol respective thin liquid crystal films around the nematic-isotropic phase transition. [ABSTRACT FROM AUTHOR]

We investigated the surface stress change of plasma-polymerized allylamine films on 2 μm thick silicon micromechanical cantilever substrates induced by ultraviolet light (UV) irradiation. Compressive surface stress was generated during the UV irradiation of the plasma-polymerized films in a dry environment, whereas tensile stress was measured in a humid environment. Fourier transform infrared spectroscopic analysis indicated two mechanisms taking place depending on the environmental conditions. These were attributed to crosslinking and oxidation reactions of the plasma polymer. UV irradiation of plasma polymerized allylamine films at defined humidity suggests a feasible method for achieving tensile and compressive surface stress patterning. [ABSTRACT FROM AUTHOR]

Studies an ohmic nanocontact to GaAs. Formation and characterization of the ohmic contacts; Determination of specific contact resistance; Demands on nanocontacts.

Demonstrates the evolution of visible photoluminescence of porous silicon films. Study of surface structure on a nanometer scale; Use of scanning tunneling microscopy; Preparation of samples under several conditions; Correlation between surface morphology and luminescence.