Atomic Sharpness Research Articles

Ripplocation and kink boundaries in graphene/copper nanolaminates: A molecular dynamics study

Over the past decade, four distinct differences have been summarized regarding ripplocation and kink boundaries in layered solids: spontaneous reversibility, atomic sharpness, delamination, and residual stress. However, according to molecular dynamics simulations, these four distinct differences are not completely applicable to the ripplocation and kink boundaries in graphene/copper nanolaminates. Contrary to conventional understanding, ripplocation boundaries can transform into kink boundaries without triggering the collective movement of other ripplocations. In addition, ripplocation boundaries can be irreversible with out-of-plane slip, leading to coherent twins in [110] orientation and stacking faults in [111] orientation. Moreover, delamination only occurs at kink boundaries rather than ripplocation boundaries. Wide-spreading residual stresses are absent for both ripplocation and kink boundaries due to plasticity. These findings broaden the current understanding of ripplocation and kink boundaries in layered solids.

Finite element analysis on the near field properties of metallic cavities with atomic sharpness

Using finite element calculations, we investigated the near field properties of two types of commonly used metal nanostructures (a tip-substrate model representing a scanning tunneling microscope type setup (TS) and a nanoparticle-on-mirror (NPoM) configuration) with atomic sharpness and extreme focusing capabilities. The spatial confinement and electric field enhancement of the local field as well as the fluorescence quantum efficiency of a model molecule (as represented by an oscillating dipole) in the cavity region of the two models were systematically studied. It was found that the TS model tends to support higher local electric field enhancement while the NPoM model can provide a more localized plasmon electric field near the nanoparticle. Calculations with the radiating model molecule indicate that both TS and NPoM can cause significant enhancements to the non-radiative decay rates at the order of 106 in the wavelength range of 500–1000 nm. The TS model shows better performance for the radiative enhancements and the resulting emission quantum yield. These results are not only helpful to improve the understanding of such important nanocavities but also supply a reference for their further applications in different areas.

Formation and crystallographic orientation of NiSi2–Si interfaces

The transport properties of novel device architectures depend strongly on the morphology and the quality of the interface between contact and channel materials. In silicon nanowires with nickel silicide contacts, NiSi2–Si interfaces are particularly important as NiSi2 is often found as the phase adjacent to the silicide–silicon interface during and after the silicidation. The interface orientation of these NiSi2–Si interfaces as well as the ability to create abrupt and flat interfaces, ultimately with atomic sharpness, is essential for the properties of diverse emerging device concepts. We present a combined experimental and theoretical study on NiSi2–Si interfaces. Interfaces in silicon nanowires were fabricated using silicidation and characterized by high-resolution (scanning) transmission electron microscopy. It is found that {111} interfaces occur in ⟨110⟩ nanowires. A tilted interface and an arrow-shaped interface are observed, which depends on the nanowire diameter. We have further modeled NiSi2–Si interfaces by density functional theory. Different crystallographic orientations and interface variations, e.g., due to interface reconstruction, are compared with respect to interface energy densities. The {111} interface is energetically most favorable, which explains the experimental observations. Possible ways to control the interface type are discussed.

A New Opportunity for 2D van der Waals Heterostructures: Making Steep-Slope Transistors.

The use of a foreign metallic cold source (CS) has recently been proposed as a promising approach toward the steep-slope field-effect-transistor (FET). In addition to the selection of source material with desired density of states-energy relation (D(E)), engineering the source:channel interface for gate-tunable channel-barrier is crucial to CS-FETs. However, conventional metal:semiconductor (MS) interfaces generally suffer from strong Fermi-level pinning due to the inevitable chemical disorder and defect-induced gap states, precluding the gate tunability of the barriers. By comprehensive materials and device modeling at the atomic scale, it is reported that 2D van der Waals (vdW) MS interfaces, with their atomic sharpness and cleanness, can be considered as general ingredients for CS-FETs. As test cases, InSe-based n-type FETs are studied. It is found that graphene can be spontaneously p-type doped along with slightly opened bandgap around the Dirac-point by interfacing with InSe, resulting in superexponentially decaying hot carrier density with increasing n-type channel-barrier. Moreover, the D(E) relations suggest that 2D transition-metal dichalcogenides and 2D transition-metal carbides are a rich library of CS materials. Graphene, Cd3 C2 , T-VTe2 , H-VTe2 , and H-TaTe2 CSs lead to subthreshold swing below 60 mV dec-1 . This work broadens the application potentials of 2D vdW MS heterostructures and serves as a springboard for more studies on low-power electronics based on 2D materials.

Sandwiched Graphene Interdiffusion Barrier for Preserving Au@Pt Atomically Thin Core@Shell Structure and the Resulting Oxygen Reduction Reaction Catalytic Activity.

The concept of a core-shell metallic structures, with a few atomic layers of the "shell" metal delineated from the "core" metal with atomic sharpness opens the door to a multitude of surface-driven materials properties that can be tuned. However, in practice, such architectures are difficult to retain due to the entropic cost of a segregated near-surface architecture, and the core and surface atoms inevitably mix through interdiffusion over time. We present here a systematic study of interdiffusion in a Pt on Au core-shell architecture and the role of an interrupting single layer of graphene sandwiched between them. The physical and chemical structure of the (near)surface is probed viamean-free-path tuned X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy (HRTEM), and electrochemistry (the oxygen reduction reaction, ORR). We find that at operating temperatures above 100 °C, there is potential for interdiffusion to occur between the primary and support metals of the core-shell catalyst system, which can diminish the catalyst activity toward ORR. The introduction of a single-layer graphene, as an interface between the core and shell metal layers, acts as a barrier that prevents unwanted surface alloying between the layered metals. HRTEM imaging shows that fully wetted Pt monolayers can be grown on a graphene template, allowing a high level of surface utilization of the catalyst material. We present how the use of graphene as a barrier to diffusion mitigates the loss of surface catalytic sites, showing much improved retention of Pt monolayer surface at elevated temperatures.

One Step Toward a New Generation of C-MOS Compatible Oxide P-N Junctions: Structure of the LSMO/ZnO Interface Elucidated by an Experimental and Theoretical Synergic Work.

Heterostructures formed by La0.7Sr0.3MnO3/ZnO (LSMO/ZnO) interfaces exhibit extremely interesting electronic properties making them promising candidates for novel oxide p-n junctions, with multifunctional features. In this work, the structure of the interface is studied through a combined experimental/theoretical approach. Heterostructures were grown epitaxially and homogeneously on 4″ silicon wafers, characterized by advanced electron microscopy imaging and spectroscopy and simulated by ab initio density functional theory calculations. The simulation results suggest that the most stable interface configuration is composed of the (001) face of LSMO, with the LaO planes exposed, in contact with the (112̅0) face of ZnO. The ab initio predictions agree well with experimental high-angle annular dark field scanning transmission electron microscopy images and confirm the validity of the suggested structural model. Electron energy loss spectroscopy confirms the atomic sharpness of the interface. From statistical parameter estimation theory, it has been found that the distances between the interfacial planes are displaced from the respective ones of the bulk material. This can be ascribed to the strain induced by the mismatch between the lattices of the two materials employed.

Atomically Sharp Crack Tips in Monolayer MoS2 and Their Enhanced Toughness by Vacancy Defects.

We combine in situ transmission electron microscopy and large-scale molecular dynamics simulations to investigate brittle fracture in 2D monolayer MoS2, revealing that cracks propagate with a tip of atomic sharpness through the preferential direction with least energy release. We find that sparse vacancy defects cause crack deflections, while increasing defect density shifts the fracture mechanism from brittle to ductile by the migration of vacancies in the strain fields into networks. The fracture toughness of defective MoS2 is found to exceed that of graphene due to interactions between the atomically sharp crack tips and vacancy clusters during propagation. These results show that monolayer 2D materials are ideal for revealing fundamental aspects of fracture mechanics not previously possible with thicker materials, similar to studies of dislocation behavior in 2D materials.

Noncontact atomic force microscopy III.

No other method has opened the door to progress in nanoscience and nanotechnology as much as the introduction of scanning probe methods did in the 1980s, since they offer a way to visualize the nanoworld. For maximum impact, however, the ability to image and manipulate individual atoms is the key. Initially, scanning tunneling microscopy was the only scanning-probe-based method that was able to achieve this resolution. Atomic force microscopy (AFM), on the other hand, was quickly developed into a versatile tool with applications ranging from materials characterization in ultrahigh vacuum and nanofabrication under ambient conditions, to biological studies in liquids, but its resolution was limited to the nanometer scale. The reason for this restriction resulted from the fact that the resolution in probe microscopy scales with the sharpness of the tip. In conventional AFM operational modes, a tip that is located at the end of a leaf spring (the so-called cantilever) is either dragged over the surface in permanent contact or gently taps the surface while vibrating, and, whichever mode is used, tips quickly blunt through either permanent or intermittent contact. Maintaining the atomic sharpness of an initially atomically sharp tip requires that the tip never touches the surface. But how can the tip know that the surface is there if it is not allowed to touch? This problem was solved in the 1990s through the realization that the attractive forces acting on the tip when it is in close proximity to the sample affect the resonance frequency of the cantilever even though it is not in actual contact with the surface. Noncontact atomic force microscopy (NC-AFM) makes use of this effect by tracking the shift of the cantilever resonance frequency due to the force field of the surface without ever establishing physical contact between the tip and sample. Much to the astonishment of many, changes induced by individual atoms turned out to induce frequency shifts that are large enough to be detected, and thus atomic-scale imaging with AFM became a reality. Since the beginnings, almost two decades ago, NC-AFM has evolved into a powerful method that is able not just to image surfaces, but also to quantify tip–sample forces and interaction potentials as well as to manipulate individual atoms on conductors, semiconductors, and insulators alike. For the community to keep track of the rapid development in the field, a series of annual international conferences, starting in Osaka, Japan in 1998, has been established. The most recent conference from this series was held in Lindau, Germany, from September 18–22, 2011. Once again, substantial progress was presented; NC-AFM is now able to quantitatively map three-dimensional force fields of surfaces with atomic resolution in ultrahigh vacuum as well as in liquids, and methodological developments add more information to the measurements, for example, through the driving of higher cantilever harmonics or the recording of tunneling currents. For this Thematic Series of the Beilstein Journal of Nanotechnology, many of the presenters from the Lindau conference agreed to submit contributions in order to assemble a series that showcases the present state of the art in the field. I would like to thank all authors who have contributed their excellent original work to this series, all referees whose promptly provided reports have provided valuable suggestions for further improvements while keeping the publication times short, and the entire NC-AFM community for supporting the open access policy of the Beilstein Journal of Nanotechnology. Udo D. Schwarz New Haven, February 2012

Oxide-free InAs(111)A interface in metal-oxide-semiconductor structure with very low density of states prepared by anodic oxidation

In this letter, we present structural, compositional, and electrical characteristics of anodic oxide layer-based metal-oxide-semiconductor (MOS) capacitors on n-type InAs(111)A, along with the effect of a thin fluorinated interfacial passivation layer. Electrochemical oxidation in acid electrolyte with addition of fluorine (NH4F) led to the formation of oxygen free well-ordered wide gap fluorinated interfacial layer at InAs(111)A with the fixed charge (Qfix) and density of interface states (Dit) in the range of (4–6) × 1010 cm−2 and (2–12) × 1010 eV−1 cm−2, respectively. We found that MOS capacitors showed excellent capacitance-voltage characteristics with very small frequency dispersion (<1% and <15 mV). Fluorinated interfacial layer consists of crystalline isostructural compound with the InAs substrate, which remains intact with the atomic smoothness and sharpness that explain unpinned behavior of the Fermi level.

Exploring atomic-scale lateral forces in the attractive regime: a case study on graphite (0001)

A non-contact atomic force microscopy-based method has been used to map the static lateral forces exerted on an atomically sharp Pt/Ir probe tip by a graphite surface. With measurements carried out at low temperatures and in the attractive regime, where the atomic sharpness of the tip can be maintained over extended time periods, the method allows the quantification and directional analysis of lateral forces with piconewton and picometer resolution as a function of both the in-plane tip position and the vertical tip–sample distance, without limitations due to a finite contact area or to stick-slip-related sudden jumps of tip apex atoms. After reviewing the measurement principle, the data obtained in this case study are utilized to illustrate the unique insight that the method offers. In particular, the local lateral forces that are expected to determine frictional resistance in the attractive regime are found to depend linearly on the normal force for small tip–sample distances.

Noncontact atomic force microscopy

No other method has opened the door to progress in nanoscience and nanotechnology as much as the introduction of scanning probe methods did in the 1980s, since they offer a way to visualize the nanoworld. For maximum impact, however, the ability to image and manipulate individual atoms is the key. Initially, scanning tunneling microscopy was the only scanning-probe-based method that was able to achieve this resolution. Atomic force microscopy (AFM), on the other hand, was quickly developed into a versatile tool with applications ranging from materials characterization in ultrahigh vacuum and nanofabrication under ambient conditions, to biological studies in liquids, but its resolution was limited to the nanometer scale. The reason for this restriction resulted from the fact that the resolution in probe microscopy scales with the sharpness of the tip. In conventional AFM operational modes, a tip that is located at the end of a leaf spring (the so-called cantilever) is either dragged over the surface in permanent contact or gently taps the surface while vibrating, and, whichever mode is used, tips quickly blunt through either permanent or intermittent contact. Maintaining the atomic sharpness of an initially atomically sharp tip requires that the tip never touches the surface. But how can the tip know that the surface is there if it is not allowed to touch? This problem was solved in the 1990s through the realization that the attractive forces acting on the tip when it is in close proximity to the sample affect the resonance frequency of the cantilever even though it is not in actual contact with the surface. Noncontact atomic force microscopy (NC-AFM) makes use of this effect by tracking the shift of the cantilever resonance frequency due to the force field of the surface without ever establishing physical contact between the tip and sample. Much to the astonishment of many, changes induced by individual atoms turned out to induce frequency shifts that are large enough to be detected, and thus atomic-scale imaging with AFM became a reality. Since the beginnings, almost two decades ago, NC-AFM has evolved into a powerful method that is able not just to image surfaces, but also to quantify tip–sample forces and interaction potentials as well as to manipulate individual atoms on conductors, semiconductors, and insulators alike. For the community to keep track of the rapid development in the field, a series of annual international conferences, starting in Osaka, Japan in 1998, has been established. The most recent conference from this series was held in Lindau, Germany, from September 18–22, 2011. Once again, substantial progress was presented; NC-AFM is now able to quantitatively map three-dimensional force fields of surfaces with atomic resolution in ultrahigh vacuum as well as in liquids, and methodological developments add more information to the measurements, for example, through the driving of higher cantilever harmonics or the recording of tunneling currents. For this Thematic Series of the Beilstein Journal of Nanotechnology, many of the presenters from the Lindau conference agreed to submit contributions in order to assemble a series that showcases the present state of the art in the field. I would like to thank all authors who have contributed their excellent original work to this series, all referees whose promptly provided reports have provided valuable suggestions for further improvements while keeping the publication times short, and the entire NC-AFM community for supporting the open access policy of the Beilstein Journal of Nanotechnology. Udo D. Schwarz New Haven, February 2012

Novel Nanoscale Tunneling Architectures for DNA Analysis

Rapid, label-free analysis of individual biopolymers, specifically of individual DNA fragments is of great importance to many areas of biology and medicine. In recent years, translocation experiments within solid-state pores and protein channels combined with ionic current blockade measurements have become the technology of choice when detection is needed at the single molecule level. For linear biopolymers such as DNA and RNA however, detection based on ionic current blockade seem to lack the signal sensitivity necessary to obtain structural information with single base resolution. Transverse (perpendicular to the helix axis) conductance measurements of DNA in nanometer-sized tunneling junctions promise current detection limits within single nucleotide resolution. Yet, the exact alignment of nanoscale electrodes in tunneling regime to a solid-state nanopore has proven to be a significant challenge. We address this shortcoming by developing a novel method for aligning nanopore and tunneling junction in a nanoscale tunneling architecture by electrochemical metal deposition. As a result, tunnelling electrodes can be fabricated with atomic sharpness and precisely aligned to the nanopore. DNA can be driven electrophoretically through the tunneling architecture and it may be possible to detect modulations in the tunneling current specific to each base in the DNA.

Microstructural studies of La10.7Ca0.3MnO3 (LCMO) films on different (110) substrates deposited by pulsed laser deposition

La0. 7Ca0. 3MnO3 (LCMO) films grown on (110) surfaces of various (pseudo-) cubic substrates, SrTiO3 (STO), LaAlO3 (LAO) and La0.3Sr0.7Al0.5Ta0.35O3 (LSAT), were studied by means of transmission electron microscopy (TEM). (110) LSAT substrate and LCMO film have the smallest lattice mismatch. (110) LAO substrate induces compressive strain while (110) STO induces tensile strain in the LCMO film. In all cases, the (010)° plane (long axis) of LCMO is parallel to the substrate surface. In the case of LSAT, the [001]° is parallel to [011] LSAT while the films on the other 2 substrates have [100]°// to the [011] substrates. Films grown on STO and LAO show an easy magnetisation when the applied magnetic field is in plane along the long-axis of the LCMO. In the case of LSAT, there is no in-plane anisotropy. The films on STO and LAO have twins as the dominant defects. The film on LSAT has less twining. High resolution TEM shows that LCMO on (110) LSAT (the least lattice mismatch) possesses a perfect interface with no misfit dislocations in the area of examination. The LCMO films on (110) LAO and (110) STO have nearly perfect interfaces with atomic sharpness but with misfit dislocations. The different in-plane crystal directions and strain distributions in the films may account for the different magnetisation behaviours of the LCMO films on the different (011) substrates.

III–V Metal–Oxide–Semiconductor Field-Effect Transistors with High κ Dielectrics

Research efforts on achieving low interfacial density of states (Dit) as well as low electrical leakage currents on GaAs-based III–V compound semiconductors are reviewed. Emphasis is placed on ultra high vacuum (UHV) deposited Ga2O3(Gd2O3) and atomic layer deposition (ALD)-Al2O3 on GaAs and InGaAs. Ga2O3(Gd2O3), the novel oxide, which was electron-beam evaporated from a gallium-gadolinium-garnet target, has, for the first time, unpinned the Fermi level of the oxide/GaAs heterostructures. Interfacial chemical properties and band parameters of valence band offsets and conduction band offsets in the oxides/III–V heterostructures are studied and determined using X-ray photoelectron spectroscopy and electrical leakage transport measurements. The mechanism of III–V surface passivation is discussed. The mechanism of Fermi-level unpinning in ALD-Al2O3ex-situ deposited on InGaAs were studied and unveiled. Systematic heat treatments under various gases and temperatures were studied to achieve low leakage currents of 10-8–10-9 A/cm2 and low Dit's in the range of (4–9)×1010 cm-2 eV-1 for Ga2O3(Gd2O3) on InGaAs. By removing moisture from the oxide, thermodynamic stability of the Ga2O3(Gd2O3)/GaAs heterostructures was achieved with high temperature annealing, which is needed for fabricating inversion-channel metal–oxide–semiconductor filed-effect transistors (MOSFET's). The oxide remains amorphous and the interface remains intact with atomic smoothness and sharpness. Device performances of inversion-channel and depletion-mode III–V MOSFET's are reviewed, again with emphasis on the devices using Ga2O3(Gd2O3) as the gate dielectric.

Defining new frontiers in electronic devices with high κ dielectrics and interfacial engineering

Ga 2O 3(Gd 2O 3), a high κ gate dielectric, ultrahigh vacuum (UHV)-deposited on GaAs and InGaAs has unpinned the Fermi level in the high-electron-mobility III–V compound semiconductors for the first time. Systematic heat treatments under various gases and temperatures were studied to achieve low leakage currents of 10 − 8 –10 − 9 A/cm 2 and low interfacial density of states ( D it's) in the range of < 10 11 cm − 2 eV − 1 . By removing moisture from the oxide, thermodynamic stability of the Ga 2O 3(Gd 2O 3)/GaAs heterostructures and the interfaces were achieved with high temperature annealing, the oxide remains amorphous and the interface remains intact with atomic smoothness and sharpness. The Fermi-level unpinning in atomic layer deposition (ALD) Al 2O 3 ex-situ deposited on InGaAs was achieved. Recent work of extremely high-quality nano-thick single crystal oxides of gamma-Al 2O 3 and bixbyite cubic Sc 2O 3 epitaxially grown on Si (111) is discussed. Interfacial manipulation is essential in giving excellent results presented in the paper. X-ray diffraction, reflectivity, and X-ray photoelectron spectroscopy using synchrotron radiation are critical in probing the interfacial properties.

III-V MOSFET's with Advanced High k Dielectrics

Ga2O3(Gd2O3), the novel oxide, which was electron-beam evaporated from a gallium- gadolinium-garnet target in UHV, has unpinned the GaAs Fermi level. Systematic heat treatments under various gases and temperatures were studied to achieve low leakage currents of 10-8 - 10-9 A/cm2 and low Dit's in the range of 4-9 x 1010 cm-2V-1 for Ga2O3(Gd2O3) on InGaAs. By removing moisture from the oxide, thermodynamic stability of the Ga2O3(Gd2O3)/GaAs hetero-structures was achieved with high temperature annealing, which is needed for fabricating inversion-channel MOSFET's. The oxide remains amorphous and the interface remains intact with atomic smoothness and sharpness. The mechanism of Fermi-level unpinning and the band parameters in atomic layer deposition (ALD)-Al2O3 ex-situ deposited on InGaAs were studied and unveiled. The work of inversion- channel, depletion- mode, and power GaAs MOSFET's using Ga2O3 (Gd2O3) as the gate dielectric was briefly reviewed.

Micromachined ultrasharp silicon and diamond-coated silicon tip as a stable field-emission electron source and a scanning probe microscopy sensor with atomic sharpness

A novel combination of silicon micromachining and deposition on silicon whiskers on (111) oriented silicon substrates shows promise for stable field emission electron emitters and sensors for scanning probe microscopy. As a field emission electron emitters show promise for high-density and high-resolution electron beam applications owing to a small (within 3°) divergence half angle of emitted electrons. Due to the sharpening of the diamond coated Si tips the threshold voltage, which is necessary to create a stable emission current (nanoampers) is decreased from the initial 1300 V to about 150 V. The diamond coating stabilizes the field-emission current as large as about 100 μA from a single tip. Training of the emitter at a current of about 10−7 A for several hours results in an additional improvement of the current stability with an additional slight current increase. Second application of vapor–liquid–solid grown silicon whiskers described here is the recent progress toward high aspect ratio (&amp;gt;100) probes and relates a technique for preparation of cantilevers for atomic force microscopy with atomic resolution on the very end of the tip. The radii are less than 2 nm for silicon tips and the angles at the ends are less than 20°. In the case of diamond, the tip radius is 10 nm.

Scanning-probe-based science and technology.

Scanning-probe-based science and technology.

New Evidence for Localized Electronic States on Atomically Sharp Field Emitters.

We have studied field emission from atomically sharp tungsten-carbide-coated $\mathrm{W}〈111〉$ tips, and from atomically sharp $\mathrm{HfC}〈100〉$ and $\mathrm{ZrC}〈100〉$ tips. We observed multiple-peaked total energy distributions from the apex atoms. These narrow and intense peaks have strong angular anisotropy, and their relative magnitudes depend on the extraction field. They suggest the presence of localized states on the atom at the apex of the tip.

Initial results on the Ag|CdO {222} interface: Atomic scale interfacial chemistry and sequencing of ordered cadmium/oxygen planes

The bonding between metal-ceramic interfaces is dependent upon the crystallographic structure and chemical composition of this heterophase interface [1-4]. In the case of metal/metal oxide interfaces, the chemistry of the interface can control the properties of the bulk material. The terminating plane of a metal oxide at a heterophase interface may be a mixed cation-oxygen, pure oxygen, or pure cation plane. For example, ab initio calculations indicate that oxygen atoms in a terminating {002} plane of MgO bond to either Ag or Ti atoms in a metallic overlayer, while the composition of the terminating MgO {002} plane remains unchanged [1,3]. High resolution electron microscopy (HREM) is used to determine the crystallographic structures of metal/metal oxide interfaces [57]. Our recent studies of the copper/magnesium oxide (222} heterophase interface -produced by internal oxidation of Cu(Mg) alloys -using conventional transmission electron microscopy (CTEM), HREM, and atom-probe field-ion microscopy (APFIM) have demonstrated the atomic scale sharpness and chemical nature of the {222} terminating planes [8-10], In particular, it is found that magnesium oxide precipitates have a cube-on-cube orientation relationship with the copper matrix, are octahedrally-shaped, and are bound by {222} planes. APFIM results demonstrate directly that the interfaces are atomically sharp, and the sequence of {222}-type planes across the interface is CulOIMg... and not CulMglO ....