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Abstract Using scanning tunneling microscopy, the influence of a thin Au layer on the diffusion of Fe adatoms and the subsequent island nucleation on a Si(111) surface is investigated. The adsorbate induces the structure that increases the surface mobility of subsequently deposited Fe atoms, resulting in the formation well-defined nanoclusters. Surprisingly, the domain walls—inherent to the reconstruction—do not influence the surface diffusion, which demonstrates that the passivation is of much more importance for the self-assembly than the surface corrugation. Using the decoupling of the diffusion and nucleationonthe surface and the reactionwiththe surface and conventional nucleation theory, the activation energy for surface diffusionE d = 0.61 eV and the critical cluster sizei = 3 are determined, which reveal the microscopic details of the diffusion and nucleation processes.

The influence of Ag-induced surface reconstructions on the formation of low-dimensional Fe-Si structures is studied using scanning tunneling microscopy. It is shown that the formation of nanostructures on the Ag-covered Si surface is possible for significantly lower temperatures compared to their formation on the bare Si surface.

We have investigated the correlation between magnetic and structural properties in exchange coupled polycrystalline CoO/Fe thin films. It has been found that an increase in interface roughness increases the exchange bias field as well as the coercivity. The magnetization reversal mechanism is also influenced by the interfacial morphology. Smooth interfaces are characterized by an asymmetric hysteresis loop, which is associated with domain wall motion for the first magnetization reversal after field cooling and spin rotation in all subsequent reversals. This asymmetry diminishes as the interface roughness increases, i.e., all magnetization reversals are dominated by spin rotation. Moreover, we have observed that the blocking temperature decreases with increasing interface roughness. We also report on a logarithmic time dependence of the magnetization which is different for both branches of the hysteresis loop of smooth CoO/Fe bilayers. [ABSTRACT FROM AUTHOR]

Scanning tunneling microscopy was used to investigate the influence of Ag-induced surface reconstructions on the formation of low-dimensional Fe-Si structures. The deposition of 1 Å Fe (i.e., 1.1 monolayer) at 300 °C on the 3 ⊗ 1-Ag, the √ 3 ⊗ √ 3-Ag, and the 7 ⊗ 7 reconstructions of the Si(111) surface results in the self-assembly of small islands. For both Ag-induced reconstructions, these islands are significantly larger compared to those formed on the Si(111)-7 ⊗ 7 surface due to an increased surface diffusion. Moreover, on the √ 3 ⊗ 3 structure, these nanodots are well separated and in between, the initial reconstruction remains unchanged. In the presence of surface steps, these islands preferentially nucleate at the lower step edge, which can be used to grow long continuous nanowires for higher Fe coverage and vicinal surfaces. Furthermore, from the phenomena such as step retraction and island/hole combinations, it is concluded that these nanostructures consist of Fe-silicide. [ABSTRACT FROM AUTHOR]

Using scanning tunneling microscopy, the influence of a thin Au layer on the diffusion of Fe adatoms and the subsequent island nucleation on a Si(111) surface is investigated. The adsorbate induces the $$\hbox{Si(111)-}\sqrt{3}\times \sqrt{3} \hbox{-Au} $$ structure that increases the surface mobility of subsequently deposited Fe atoms, resulting in the formation well-defined nanoclusters. Surprisingly, the domain walls—inherent to the $$\sqrt{3} \times \sqrt{3} \hbox{-Au} $$ reconstruction—do not influence the surface diffusion, which demonstrates that the passivation is of much more importance for the self-assembly than the surface corrugation. Using the decoupling of the diffusion and nucleation on the surface and the reaction with the surface and conventional nucleation theory, the activation energy for surface diffusion Ed = 0.61 eV and the critical cluster size i = 3 are determined, which reveal the microscopic details of the diffusion and nucleation processes. [ABSTRACT FROM AUTHOR]

The experimental study of the bonding geometry of a (100)Ge surface exposed to H2S in the gas phase at 330 °C shows that 1 ML S coverage with (2×1) surface reconstruction can be achieved. The amount of S on the Ge surface and the observed surface periodicity can be explained by the formation of disulfide bridges between Ge–Ge dimers on the surface. First-principles molecular dynamics simulations confirm the preserved (2×1) reconstruction after dissociative adsorption of H2S molecules on a (100)Ge (2×1) surface, and predict the formation of (S–H)–(S–H) inter-Ge dimer bridges, i.e., disulfide bridges interacting via hydrogen bonding. The computed energy band gap of this atomic configuration is shown to be free of surface states, a very important finding for the potential application of Ge in future high performance integrated circuits. [ABSTRACT FROM AUTHOR]

We have gained insight into the internal degree of atomic disorder in isolated size-selected Fe nanoparticles (NPs) (~2-6 ran in size) supported on Si02/Si(l 11) and Al2O3(0001) from precise measurements of the low-energy (low-£) part of the phonon density of states [PDOS, g(E)] via 57Fe nuclear resonant inelastic x-ray scattering (NRIXS) combined with transmission electron microscopy (TEM) measurements. An intriguing size-dependent trend was observed, namely, an increase of the low-£ excess density of phonon states (as compared to the PDOS of bulk bcc Fe) with increasing NP size. This is unexpected, since usually the enhancement of the density of low-E phonon modes is attributed to low-coordinated atoms at the NP surface, whose relative content increases with decreasing NP size due to the increase in the surface-to-volume ratio. Our NPs are covered by a Ti-coating layer, which essentially restores the local neighborhood of surface Fe atoms towards bulk-like coordination, reducing the surface effect. Our data can be qualitatively explained by the existence of low-coordinated Fe atoms located at grain boundaries or other defects with structural disorder in the interior of the large NPs (~3-6 nm), while our small NPs (~2 nm) are single grain and, therefore, characterized by a higher degree of structural order. This conclusion is corroborated by the observation of Debye behavior at low energy [g(E) ~ E" with n ~ 2] for the small NPs, but non-Debye behavior (with n ~ 1.4) for the large NPs. The PDOS was used to determine thermodynamic properties of the Fe NPs. Finally, our results demonstrate that, in combination with TEM, NRIXS is a suitable technique to investigate atomic disorder/defects in NPs. We anticipate that our findings are universal for similar NPs with bcc structure. [ABSTRACT FROM AUTHOR]

This study presents a systematic investigation of the thermodynamic properties of free and γ-Al2O3-supported size-controlled Pt nanoparticles (NPs) and their evolution with decreasing NP size. A combination of in situ extended x-ray absorption fine-structure spectroscopy (EXAFS), ex situ transmission electron microscopy (TEM) measurements, and NP shape modeling revealed (i) a cross over from positive to negative thermal expansion with decreasing particle size, (ii) size- and shape-dependent changes in the mean square bond-projected bond-length fluctuations, and (iii) enhanced Debye temperatures (ΘD, relative to bulk Pt) with a bimodal size-dependence for NPs in the size range of ~0.8-5.4 nm. For large NP sizes (diameter d > 1.5 nm) ΘD was found to decrease toward ΘD of bulk Pt with increasing NP size. For NPs ≤ 1 nm, a monotonic decrease of ΘD was observed with decreasing NP size and increasing number of low-coordinated surface atoms. Our density functional theory calculations confirm the size- and shape-dependence of the vibrational properties of our smallest NPs and show how their behavior may be tuned by H desorption from the NPs. The experimental results can be partly attributed to thermally induced changes in the coverage of the adsorbate (H2) used during the EXAFS measurements, bearing in mind that the interaction of the Pt NPs with the stiff, high-melting temperature γ-Al2O3 support may also play a role. The calculations also provide good qualitative agreement with the trends in the mean square bond-projected bond-length fluctuations measured via EXAFS. Furthermore, they revealed that part of the ΘD enhancement observed experimentally for the smallest NPs (d ≤ 1 nm) might be assigned to the specific sensitivity of EXAFS, which is intrinsically limited to bond-projected bond-length fluctuations. [ABSTRACT FROM AUTHOR]

Scanning Spreading Resistance Microscopy is a well-established technique for obtaining quantitative two- and three-dimensional carrier profiles in semiconductor devices with sub-nm spatial resolution. However, for sub-100 nm devices, the use of focused ion beam becomes inevitable for exposing the region of interest on a sample cross section. In this work, we investigate the impact of the focused ion beam milling on spreading resistance analysis and we show that the electrical effect of the focused ion beam extends far beyond the amorphous region and depends on the dopant concentration, ion beam energy, impact angle, and current density. For example, for dopant concentrations between 1.0 × 10 20 and 1.5 × 10 16  cm -3 we observe dopant deactivation at least between 23 and 175 nm for a glancing 30 keV ion beam. Further, we show that dopant deactivation is caused by defect diffusion during milling and is not directly impacted by the presence of Gallium in the sample. Later, we also discuss potential ways to mitigate these effects.

The ability to develop ferroelectric materials using binary oxides is critical to enable novel low-power, high-density non-volatile memory and fast switching logic. The discovery of ferroelectricity in hafnia-based thin films, has focused the hopes of the community on this class of materials to overcome the existing problems of perovskite-based integrated ferroelectrics. However, both the control of ferroelectricity in doped-HfO 2 and the direct characterization at the nanoscale of ferroelectric phenomena, are increasingly difficult to achieve. The main limitations are imposed by the inherent intertwining of ferroelectric and dielectric properties, the role of strain, interfaces and electric field-mediated phase, and polarization changes. In this work, using Si-doped HfO 2 as a material system, we performed a correlative study with four scanning probe techniques for the local sensing of intrinsic ferroelectricity on the oxide surface. Putting each technique in perspective, we demonstrated that different origins of spatially resolved contrast can be obtained, thus highlighting possible crosstalk not originated by a genuine ferroelectric response. By leveraging the strength of each method, we showed how intrinsic processes in ultrathin dielectrics, i.e., electronic leakage, existence and generation of energy states, charge trapping (de-trapping) phenomena, and electrochemical effects, can influence the sensed response. We then proceeded to initiate hysteresis loops by means of tip-induced spectroscopic cycling (i.e., "wake-up"), thus observing the onset of oxide degradation processes associated with this step. Finally, direct piezoelectric effects were studied using the high pressure resulting from the probe's confinement, noticing the absence of a net time-invariant piezo-generated charge. Our results are critical in providing a general framework of interpretation for multiple nanoscale processes impacting ferroelectricity in doped-hafnia and strategies for sensing it.

Quantitative scanning spreading resistance microscopy is currently a powerful method for carrier profiling in scaled nanoelectronic devices. Faced with the further reduction of dimensions and increasing architecture complexity, a force modulation method was developed to address the challenges associated with parasitic series resistances. Called fast Fourier transform scanning spreading resistance microscopy, the method has been shown to increase dynamic range when profiling Si devices and retains the doping contrast even in the presence of a series resistance. In this work we systematically investigate the potential of fast Fourier transform scanning spreading resistance microscopy for Ge, GaAs, InP, and InGaAs, presenting a quantitative comparison with Si as well as a more in-depth understanding of the capabilities and limitations of the method. Our results show that both GaAs and InP greatly benefit, with a significantly larger dynamic range and the ability to filter undesired series resistances. Doping concentration contrast in the presence of a series resistance can also be maintained in Ge but with high noise. For InGaAs there are only minor benefits. These findings prove that fast Fourier transform scanning spreading resistance microscopy is a valuable extension to regular scanning spreading resistance microscopy for more accurate carrier profiling in Si and non-Si materials, especially in architectures where parasitic series resistances are present., (Copyright © 2019 Elsevier B.V. All rights reserved.)

Two-dimensional transition metal dichalcogenides have been the focus of intense research for their potential application in novel electronic and optoelectronic devices. However, growth of large area two-dimensional transition metal dichalcogenides invariably leads to the formation of grain boundaries that can significantly degrade electrical transport by forming large electrostatic barriers. It is therefore critical to understand their effect on the electronic properties of two-dimensional semiconductors. Using MoS 2 as an example material, we are able to probe grain boundaries in top and buried layers using conductive atomic force microscopy. We find that the electrical radius of the grain boundary extends approximately 2 nm from the core into the pristine material. The presence of grain boundaries affects electrical conductivity not just within its own layer, but also in the surrounding layers. Therefore, electrical grain size is always smaller than the physical size, and decreases with increasing thickness of the MoS 2 . These results signify that the number of layers in synthetically grown 2D materials must ideally be limited for device applications.

For the very first time, atomic force microscopy is used to determine quantitatively the 3-dimensional shape of an atom probe tip, which is key towards improved accuracy and understanding of artefacts in atom probe tomography. We have successfully measured by atomic force microscopy the apex and shank region of 3 different atom probe tips, of which two show (severe) deviations from a hemisphere due to either non-uniform laser light absorption or the presence of two different materials. Clearly, our method which overcomes the challenge of aligning two very sharp tips on top of each other, offers new pathways to study physical mechanisms in (laser-assisted) atom probe. It represents an important step towards improved reconstruction algorithms as the image formation in atom probe tomography is based on the intricate link between the tip shape (down to the atomic level), the electric field distribution and the ions' flight path towards the detector. Further on, present reconstruction algorithms solely account for a hemispherical tip shape, which does not hold true for most applications and results in complex artefacts. Therefore our method is an attractive novel approach to assess the 3D tip shape., (Copyright © 2018 Elsevier B.V. All rights reserved.)

The rapid cadence of MOSFET scaling is stimulating the development of new technologies and accelerating the introduction of new semiconducting materials as silicon alternative. In this context, 2D materials with a unique layered structure have attracted tremendous interest in recent years, mainly motivated by their ultra-thin body nature and unique optoelectronic and mechanical properties. The development of scalable synthesis techniques is obviously a fundamental step towards the development of a manufacturable technology. Metal-organic chemical vapor deposition has recently been used for the synthesis of large area TMDs, however, an important milestone still needs to be achieved: the ability to precisely control the number of layers and surface uniformity at the nano-to micro-length scale to obtain an atomically flat, self-passivated surface. In this work, we explore various fundamental aspects involved in the chemical vapor deposition process and we provide important insights on the layer-dependence of epitaxial MoS 2 film's structural properties. Based on these observations, we propose an original method to achieve a layer-controlled epitaxy of wafer-scale TMDs.

Wear mechanisms including fracture and plastic deformation at the nanoscale are central to understand sliding contacts. Recently, the combination of tip-induced material erosion with the sensing capability of secondary imaging modes of AFM, has enabled a slice-and-view tomographic technique named AFM tomography or Scalpel SPM. However, the elusive laws governing nanoscale wear and the large quantity of atoms involved in the tip-sample contact, require a dedicated mesoscale description to understand and model the tip-induced material removal. Here, we study nanosized sliding contacts made of diamond in the regime whereby thousands of nm 3 are removed. We explore the fundamentals of high-pressure tip-induced material removal for various materials. Changes in the load force are systematically combined with AFM and SEM to increase the understanding and the process controllability. The nonlinear variation of the removal rate with the load force is interpreted as a combination of two contact regimes each dominating in a particular force range. By using the gradual transition between the two regimes, (1) the experimental rate of material eroded on each tip passage is modeled, (2) a controllable removal rate below 5 nm/scan for all the materials is demonstrated, thus opening to future development of 3D tomographic AFM.

An in-depth understanding of the fundamental structure of catalysts during operation is indispensable for tailoring future efficient and selective catalysts. We report the evolution of the structure and oxidation state of ZrO(2)-supported Pd nanocatalysts (∼5 nm) during the in situ reduction of NO with H(2) using X-ray absorption fine-structure spectroscopy and X-ray photoelectron spectroscopy. Prior to the onset of the reaction (≤120 °C), a NO-induced redispersion of our initial metallic Pd nanoparticles over the ZrO(2) support was observed, and Pd(δ+) species were detected. This process parallels the high production of N(2)O observed at the onset of the reaction (>120 °C), while at higher temperatures (≥150 °C) the selectivity shifts mainly toward N(2) (∼80%). Concomitant with the onset of N(2) production, the Pd atoms aggregate again into large (6.5 nm) metallic Pd nanoparticles, which were found to constitute the active phase for the H(2)-reduction of NO. Throughout the entire reaction cycle, the formation and stabilization of PdO(x) was not detected. Our results highlight the importance of in situ reactivity studies to unravel the microscopic processes governing catalytic reactivity.

Unraveling the complex interaction between catalysts and reactants under operando conditions is a key step toward gaining fundamental insight in catalysis. We report the evolution of the structure and chemical composition of size-selected micellar Pt nanoparticles (∼1 nm) supported on nanocrystalline γ-Al(2)O(3) during the catalytic oxidation of 2-propanol using X-ray absorption fine-structure spectroscopy. Platinum oxides were found to be the active species for the partial oxidation of 2-propanol (<140 °C), while the complete oxidation (>140 °C) is initially catalyzed by oxygen-covered metallic Pt nanoparticles, which were found to regrow a thin surface oxide layer above 200 °C. The intermediate reaction regime, where the partial and complete oxidation pathways coexist, is characterized by the decomposition of the Pt oxide species due to the production of reducing intermediates and the blocking of O(2) adsorption sites on the nanoparticle surface. The high catalytic activity and low onset reaction temperature displayed by our small Pt particles for the oxidation of 2-propanol is attributed to the large amount of edge and corner sites available, which facilitate the formation of reactive surface oxides. Our findings highlight the decisive role of the nanoparticle structure and chemical state in oxidation catalytic reactions., (© 2011 American Chemical Society)