Monolayer Thick Films Research Articles

Thermal diffusion of Mn through GaAs overlayers on (Ga, Mn)As

Thermally stimulated diffusion of Mn through thin layers of GaAs has been studied byx-ray photoemission. (Ga, Mn)As samples with 5 at% Mn were capped with 4, 6 and8 monolayer (ML) GaAs, and Mn diffusing through the GaAs was trapped on the surface bymeans of amorphous As. It was found that the out-diffusion is completely suppressed for an8 ML thick GaAs film. The short diffusion length is attributed to an electrostatic barrierformed at the (Ga, Mn)As/GaAs interface.

Negative ions, energy loss, and electron emission during grazing scattering of fast H and He atoms from a clean and oxidized NiAl(1 1 0) surface

Negative ion fractions, projectile energy loss, and the emission of electrons is studied for grazing scattering of hydrogen and helium atoms/ions from a clean and oxidized NiAl(1 1 0) surface. Making use of translation energy spectroscopy and the coincident detection of the number of emitted electrons we have studied the electronic interaction mechanisms for the change from a clean metal target to an insulator surface via the preparation of a well defined ultrathin alumina film on top of the metal substrate. We find that already for a monolayer thick oxide film the characteristic different features of electronic processes for the surface of an insulator crystal are present.

The resistance of single atomic steps in ultrathin Pb nanowires on Si(557)

We studied the local electronic transport properties of a monolayer thick Pb wire by local potentiometry with the tip of a tunneling microscope. 50-nm-wide wires on bare Si(557) were generated by direct writing with an electron beam in an ultrathin film of SiO2 using the process of electron-beam-induced selective stimulated thermal desorption of oxygen (EBSTD) in combination with a shadow-mask technique and macroscopic TiSi2 contacts. The resistivity of this wire agrees well with expectations derived from anisotropic monolayer thick Pb films on Si(557). Although small Pb clusters nucleated during annealing and desorption of excess Pb, they had a negligible effect on the local resistive properties of the wire. Steps in the substrate of atomic height apparently do not interrupt the conducting path, but due to local scattering at step edge states increase the local resistivity by more than one order of magnitude.

Thermodynamic and structural studies of mixed monolayers: Mutual mixing of DPPC and DPPG with DoTAP at the air–water interface

Phospholipid monomolecular films at the air–water interface are useful model membranes to understand miscibility among various components. Surface pressure ( π)–area ( A) isotherms of pure and mixed monolayers of dioleoyltrimethylammonium propane (DoTAP)–dipalmitoylphosphatidylcholine (DPPC) and DoTAP–dipalmitoyphosphatidylglycerol (DPPG) were constructed using a surface balance. DPPC and DPPG produced isotherms as expected and reported earlier. DoTAP, an unsaturated lipid, demonstrated a continuous π– A isotherm. Associative interactions were identified in DPPC–DoTAP mixtures compared to the pure components, while DPPG–DoTAP mixtures showed repulsive interaction up to an equimolar ratio. Compression moduli of the monolayers revealed that DPPC–DoTAP mixtures had increasing stability with increasing surface pressure, but addition of DoTAP to DPPG showed instability at low and intermediate concentrations. In both cases increased stability was returned at higher X DoTAP values and surface pressures. Lipid monolayer film thickness values, determined on a gold coated glass substrate by surface plasmon resonance spectroscopy (SPR), indicated a systematic change in height profile for DPPC–DoTAP mixtures with increasing X DoTAP. However, DPPG–DoTAP mixed monolayer systems demonstrated a biphasic response. The SPR data were in excellent agreement with our interpretation of the structure of solid supported lipid monolayers.

Structure of ultrathin Pd films determined by low-energy electron microscopy and diffraction

Palladium (Pd) films have been grown and characterized in situ by low-energy electron diffraction (LEED) and microscopy in two different regimes: ultrathin films 2–6 monolayers (ML) thick on Ru(0001), and ∼20 ML thick films on both Ru(0001) and W(110). The thinner films are grown at elevated temperature (750 K) and are lattice matched to the Ru(0001) substrate. The thicker films, deposited at room temperature and annealed to 880 K, have a relaxed in-plane lattice spacing. All the films present an fcc stacking sequence as determined by LEED intensity versus energy analysis. In all the films, there is hardly any expansion in the surface-layer interlayer spacing. Two types of twin-related stacking sequences of the Pd layers are found on each substrate. On W(110) the two fcc twin types can occur on a single substrate terrace. On Ru(0001) each substrate terrace has a single twin type and the twin boundaries replicate the substrate steps.

Microstructure of a Cu film grown on bcc Ta (100) by large-scale molecular-dynamics simulations

Molecular-dynamics simulations using embedded atom method potentials were carried out to study the growth and subsequent annealing of a 6.3 monolayer thick Cu film on a bcc-Ta (100) substrate of a very large area $(100\ifmmode\times\else\texttimes\fi{}100\text{ }{\text{nm}}^{2})$. The purpose is to obtain, for this typical example of a crystallographically incompatible system, atomic-level insight into the microstructural evolution of the film/substrate interface and the film itself, with the limiting influence of in-plane boundary conditions kept to a minimum. It is found that the first Cu plane grows heteroepitaxially on the bcc-Ta (100) surface. The second Cu plane is the most interesting plane; it grows in the form of a pattern of 26-atom misfit supercells which after prolonged film deposition relax energetically, by breaking up in groups and forming $30\text{ }\text{\AA{}}$ wide in-plane island strips separated by fcc $(1/6)⟨211⟩$ vectors. The distorted monolayer represents a way of enabling epitaxy between very different crystal structures without introducing misfit dislocations. The third and higher Cu planes are fcc (111) planes. The Cu film is polycrystalline, starting from the second plane up, with two different in-plane crystal orientations. The influence of a one-monolayer Ta terrace on the substrate is minimal. Upon annealing the film at elevated temperatures, a tendency toward island coarsening and agglomeration is observed, but a full three-dimensional island formation such as found experimentally is not observed. Compared to an earlier simulation on a smaller substrate area $(18\ifmmode\times\else\texttimes\fi{}18\text{ }{\text{nm}}^{2})$, the current simulations have produced a much richer microstructure. The present results therefore warn against the use of too small simulation domains when complex strain fields can be expected.

Surface Enhanced Vibrational Spectroscopy of Proteins with Plasmonic Nanoantenna Arrays

AbstractInfrared absorption spectroscopy is a powerful tool for structural and functional studies of biomolecules. The technique enables direct access to the vibrational fingerprints of molecular bonds in the mid-infrared spectral region (3-20μm). Although intrinsic absorption cross-sections are nearly ten orders of magnitude greater than corresponding Raman cross-sections, they are still small in comparison with those of fluorescent molecules. Sensitivity improvements are therefore required for the method to be applicable to single molecule / molecular layer studies. In this work, we demonstrate the use of lithographically patterned arrays of nanoantennas to enhance the absorption signature of the protein amide-I and II backbone vibrations. Strong absorption signals from monolayer thickness films are obtained. By arranging ensembles of tailored antennas in specific lattices, higher quality factor resonances and increased near-field intensities are possible. These features are leveraged to obtain 104-105 fold signal enhancements and the direct measurement of vibrational spectra of proteins at zepto-mole sensitivity levels.

Measuring the magnetization of three monolayer thick Co islands and films by x-ray dichroism

Co islands and films are characterized by x-ray magnetic circular dichroism photoemission electron microscopy. The spatial resolution capabilities of the technique together with atomic growth control permit obtaining perfectly flat triangular islands with a given thickness (3 ML), very close to an abrupt spin-reorientation transition. The magnetic domain configurations are found to depend on island size: while small islands can be magnetized in a single-domain state, larger islands show more complex patterns. Furthermore, the magnetization pattern of the larger islands presents a common chirality. By means of dichroic spectromicroscopy at the $\text{Co}\text{ }L$ absorption edges, an experimental estimate of the ratio of the spin and orbital magnetic moment for three monolayer thick films is obtained.

Molecular Arrangement in Langmuir and Langmuir−Blodgett Films of a Mesogenic Bent-Core Carboxylic Acid

A different alternative to previous research on Langmuir and Langmuir-Blodgett (LB) films of bent-core liquid crystals is reported in this work. A bent-shaped molecule wearing a terminal carboxylic group has been used to obtain monomolecular films with their long molecular axis almost perpendicular to the aqueous surface. Langmuir films at the air-liquid interface (pH=9) have been characterized by a combination of surface pressure and surface potential versus area per molecule isotherms, Brewster angle microscopy, and ultraviolet reflection spectroscopy. A condensed phase is reached at surface pressures up to 20 mN x m-1. In this condensed phase, molecules are packed forming H-aggregates with a well-defined molecular orientation. Langmuir films have been transferred onto quartz and silicon substrates and characterized by means of UV-vis spectroscopy and XRR. The transference is Z-type, with a constant deposition of the monolayers. The total LB monolayer film thickness is evaluated to be about 5.8 nm, which is in good agreement with the deduced orientation at the air-liquid interface as well as with the lamellar order observed within the solid obtained by cooling the sample from the mesophase.

Effect of humidity on the interactions between probes and ultra-thin liquid lubricant films in an environmentally controlled atomic force microscope

To achieve a magnetic recording density of 1 Tb/in2, the required head-disk spacing is expected to be less than 2–3 nm. However, a critical issue in achieving such an ultra-low spacing is the dynamic instability of the head-disk interface (HDI), that is, the experimentally observed hysteresis of the flying sliders. It is important to clarify the interactive forces between the sliders and disk surfaces in a nanometer HDI. In addition, the adhesive forces between the thermally protruding magnetic head writer pole tip, which exists in the most commercially available recording head, and the lubricant film have also become very important. Therefore, in this study, the interactions between the probes of an environmentally controlled atomic force microscope (AFM) and ultra-thin liquid lubricant films were studied as a model experiment, resulting in the acquisition of fundamental knowledge related to the adhesive forces as well as the slider hysteresis. Subsequently, the effect of humidity on the force curve characteristics was investigated by varying the lubricant film thickness. As a result, it was found that the probe touchdown occurred earlier as the humidity increased, with its driving force suggested to be the force of the lubricant meniscus formed on the AFM probe. The probe touchdown occurred earlier for lubricants with a higher mobility. The adhesive force gradually increased with the humidity and was at its minimum for a one monolayer film thickness, independent of the lubricant materials. On the other hand, the elongation of the lubricant meniscus increased with the humidity. Therefore, the probe takeoff occurred later as the humidity increased. Accordingly, the hysteresis increased with the humidity and was mainly dependent on the elongation of the meniscus.

On the determination of Poisson’s ratio of stressed monolayer and bilayer submicron thick films

In this paper, the bulge test is used to determine the mechanical properties of very thin dielectric membranes. Commonly, this experimental method permits to determine the residual stress (σ0) and biaxial Young’s modulus (E/(1 − υ)). Associating square and rectangular membranes with different length to width ratios, the Poisson’s ratio (υ) can also be determined. LPCVD Si3N4 monolayer and Si3N4/SiO2 bilayer membranes, with thicknesses down to 100 nm, have been characterized giving results in agreement with literature for Si3N4, E = 212 ± 14 GPa, σ0 = 420 ± 8 MPa and υ = 0.29.

Role of heat flow direction, monolayer film thickness, and periodicity in controlling thermal conductivity of a Si–Ge superlattice system

Superlattices are considered one of the most promising material systems for nanotechnological applications in fields such as high figure of merit (ZT) thermoelectrics, microelectronics, and optoelectronics owing to the possibility that these materials could be tailored to obtain desired thermal properties. Factors that could be adjusted for tailoring the thermal conductivity of the superlattices include the monolayer film thickness, periodicity, heat flow direction, straining, and temperature of operation. In the presented study, nonequilibrium molecular dynamics (NEMD) simulations are performed to obtain an understanding of the effect of such factors on the thermal conductivity of Si–Ge superlattices at three different temperatures (400, 600, and 800 K). The NEMD simulations are performed using Tersoff bond-order potential. The thermal conductivity is found to increase with an increase in the number of periods as well as with the increase in the period thickness. The dependence of thermal conductivity on the direction of heat flow is found to be sensitive to the extent of acoustic mismatch at the interface (i.e., heat flowing from Si to Ge versus heat flowing from Ge to Si in a single period). Superlattices with Ge–Si interfaces (heat flows from Ge monolayer to Si monolayer in a period) are found to have lower thermal conductivity than superlattices with Si–Ge interfaces (heat flows from Si monolayer to Ge monolayer in a period). The superlattices thermal conduction, therefore, can be considered to have a characteristic somewhat similar to a thermal diode. Both compressive and tensile strains are observed to be an important factor in tailoring the thermal conductivity of the analyzed superlattices. Particularly, straining can help in reducing the thermal conductivity. The influence of straining is found to increase with increasing period thickness and periodicity.

Perpendicular magnetic anisotropy of ultrathin FeCo alloy films on Pd(0 0 1) surface: First principles study

Using the full potential linearized augmented plane wave (FLAPW) method, thickness dependent magnetic anisotropy of ultrathin FeCo alloy films in the range of 1 monolayer (ML) to 5 ML coverage on Pd(0 0 1) surface has been explored. We have found that the FeCo alloy films have close to half metallic state and well-known surface enhancement in thin film magnetism is observed in Fe atom, whereas the Co has rather stable magnetic moment. However, the largest magnetic moment in Fe and Co is found at 1 ML thickness. Interestingly, it has been observed that the interface magnetic moments of Fe and Co are almost the same as those of surface elements. The similar trend exists in orbital magnetic moment. This indicates that the strong hybridization between interface FeCo alloy and Pd gives rise to the large magnetic moment. Theoretically calculated magnetic anisotropy shows that the 1 ML FeCo alloy has in-plane magnetization, but the spin reorientation transition (SRT) from in-plane to perpendicular magnetization is observed above 2 ML thickness with huge magnetic anisotropy energy. The maximum magnetic anisotropy energy for perpendicular magnetization is as large as 0.3 meV/atom at 3 ML film thickness with saturation magnetization of 2.36 μ B . Besides, the calculated X-ray magnetic circular dichroism (XMCD) has been presented.

The influence of surface roughness on electrical conductance of thin Cu films: An ab initio study

First-principles calculations show that atomic-scale surface roughness dramatically affects the electrical conductivity of thin films. Atomic clusters, 1–3 atoms high, deposited on the flat Cu(001) surface of an 11 monolayer thick film lead to a 30−40% reduction of its conductance. This is attributed to the destruction of isotropic Fermi surface sheets. We provide a simple parametrized formula, correlating the size of the surface added structures to the film conductance, and also demonstrate that Ta and Al surface monolayers on rough Cu surfaces cause a conductance decrease and increase, respectively.

Magnetization reversal of Fe ultrathin film on Cu(100)

The magnetization reversal of Fe/Cu(100) ultrathin films grown at room temperature is investigated by using an in situ magneto-optical Kerr effect polarimeter with a magnet that can rotate in a plane of incidence. There occur spin reorientation transitions from out-of-plane to in-plane magnetizations in 8 and 12 monolayers (ML) thick iron films. The coercive fields are observed to be proportional to the reciprocal of the cosine with respect to the easy axis, suggesting that the domain-wall displacement plays a main role in the magnetization reversal process.

Ab initio modeling of TiO2 nanosheets

We present density functional calculations on 1–6 monolayer (ML) thick TiO2 films peeled off from the main low-index surfaces of anatase. The structure of the films is optimized both by constraining the lattice constants to those of bulk anatase, and by allowing them to relax. It is found that the stability order of the films does not follow that of the surfaces from which they are derived, and does not increase monotonously with film thickness. Furthermore, relaxing the lattice constants can induce large modifications in the film structure. In particular, two anomalously stable films are found. One derives from the 2 ML (001) film, and rearranges to a lepidocrocite-TiO2 nanosheet. The other one derives from a 4 ML (101) film, and gives rise to a novel phase, where all the Ti ions are fivefold coordinated.

Optical Characterization of Thiolate Self-Assembled Monolayers on Au(111)

We have investigated the optical response of thiolate self-assembled monolayers (SAMs) deposited from the liquid phase on well characterized, (111)-textured gold films based on the use of in situ and ex situ optical spectroscopic ellipsometry. We considered SAMs formed by several molecules with thiol functionality, focusing on the octadecanethiol (C18) SAM model system. We were able to show the tiny spectroscopic variations induced by the monolayer thick films with great reproducibility and high signal-to-noise ratio. We identified spectral features related to the alkyl chain and to the S−Au interface, providing a reliable spectral “fingerprint” of the formation of densely packed thiolate layers. By comparing data with simulations based on several effective models developed within the framework of Fresnel approach, we identified the main optical features related to the thiolate interface and in particular an absorption band whose spectral weight increases regularly from 500 nm toward the IR limit. We also...

Structure determination of monolayer-by-monolayer grownLa1−xSrxMnO3thin films and the onset of magnetoresistance

Surface x-ray diffraction was used to determine the atomic structures of ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{MnO}}_{3}$ thin films, grown monolayer by monolayer on ${\mathrm{SrTiO}}_{3}$ by pulsed laser deposition. Structures for one-, two-, three-, four-, six-, and nine-monolayer-thick films were solved using the Coherent Bragg rod analysis phase-retrieval method and subsequent structural refinement. Four important results were found. First, the out-of-plane lattice constant is elongated across the substrate-film interface. Second, the transition from substrate to film is not abrupt, but proceeds gradually over approximately three unit cells. Third, Sr segregates towards the topmost monolayer of the film: we determined a Sr-segregation enthalpy of $\ensuremath{-}15\phantom{\rule{0.3em}{0ex}}\mathrm{kJ}∕\mathrm{mol}$ from the occupation parameters. Finally, the electronic bandwidth $W$ was used to explain the onset of magnetoresistance for films of nine or more monolayers thickness. Resistivity measurements of the nine monolayer-thick film confirm magnetoresistance and the presence of a dead layer with mostly insulating properties.

Unique activity of Pd monomers: hydrogen evolution at AuPd(111) surface alloys

Well-defined Au/Pd(111) alloy films have been prepared on a Ru(0001) substrate by electrochemical metal deposition and subsequent heating up to 700 degrees C. The electrochemical behaviour of the 20 monolayers thick epitaxially-grown films is in excellent agreement with both equilibrium surface composition and distribution for Au/Pd alloys on Mo(110) as previously reported (D. W. Goodman et al., J. Phys. Chem., 2005, B109, 18535). The electrocatalytic activity of the AuPd(111) surface alloys was studied for the hydrogen evolution in 0.1 M H(2)SO(4) as a function of surface composition. Maximum activities were found for Pd fractions of 0.2 +/- 0.1, where the population of Pd atoms surrounded by Au has its maximum. These Pd monomers are found to be about 20 times more active than Pd atoms in the Pd overlayer.

Smoothening of ultrathin Fe films grown on GaAs(100) observed by scanning tunneling microscopy and Brillouin light scattering

We report on an in situ scanning tunneling microscopy study of the growth of Fe on the GaAs(100)-pseudo-$4\ifmmode\times\else\texttimes\fi{}6$ reconstructed surface in the thickness range of 1--4 ML (monolayers). We observe that at 1 ML, the Fe islands have nucleated on the As dimer rows, and that subsequent growth is primarily directed along the $[0\overline{1}1]$ direction. Initially, the islands are well separated, but as more Fe is deposited, the islands start to coalesce in both the [011] and $[0\overline{1}1]$ directions. Percolation probably occurs between 2 and 3 ML. Scanning tunneling microscopy studies of a nominally 2 ML thick film over a period of $30\phantom{\rule{0.3em}{0ex}}\mathrm{h}$ reveal that the islands relax primarily growing in the $[0\overline{1}1]$ direction, reducing the roughness of the film. The smoothening of the film is also corroborated by Brillouin light scattering measurements on slightly thicker films of 4.25 and 6.2 ML as evidenced by an increase in the spin-wave frequency and a decrease of the linewidth over a period of $11\phantom{\rule{0.3em}{0ex}}\mathrm{h}$.