Number Of Atomic Layers Research Articles

Determination of Single- and Multi-Component Nanoparticle Sizes by X-ray Absorption Spectroscopy

An approach to estimate the size of quasi-spherical nanoparticle systems composed of atoms crystalizing in face centered cubic lattice using the data from X-ray absorption spectroscopy is introduced. For monoatomic clusters, the number of atomic layers surrounding the X-ray-absorbing atom m is evaluated from the coordination numbers, and the size of the particle calculated as the diameter of a sphere having the same volume as the cuboctahedron with the edge length of m atoms, assuming complete atomic layer structure. The particle size estimated by the approach agrees well with the experimental data obtained by other methods for clusters up to 5 nm. For bimetallic systems, coordination numbers of one component to the other in various inner-structured mixtures were derived. The interatomic distance of the solid solution is approximated to the weighted mean of the two constituents’ lattice parameters and these mean values were used to estimate the particle size. The method could be applied to core-shell structures of bimetallic systems, as well as to three-component particles using reasonable approximations, and the particle sizes are estimated in a similar manner. The diameters of polyatomic clusters estimated by this approach were shown to closely follow the experimental data obtained by independent techniques.

Graphite oxide electrical sensors are able to distinguish single nucleotide polymorphisms in physiological buffers

Atomically flat materials based on graphene present unique opportunities for new technologies and are expected to standardize in near future for their properties based on size, shape and number of atomic layers. On the thicker end of the spectrum, graphite oxides (GrO) present a unique set of material properties in the family of graphene-based 2D materials. In this work, an optimized chemical sourcing and bottom-up nanofabrication approach is utilized for scalable fabrication of a highly sensitive field-effect based biodetector platform, deployable in concentrated liquid medium. GrO based biodetectors displaying very low variations in device-to-device sensor characteristics, were deployed for label-free detection of DNA concentrations from 5 pM to 5 nM in physiological buffer. Furthermore, charge-transport behaviour of GrO devices in electrochemical gate configuration was compared with that of the state-of-the-art graphene based devices. A comparative model for GrO-electrolyte interface is discussed based on the recent developments in this field and elucidate the role surface functional layers play in influencing the field-effect and biosensor performance of GrO based devices. The biodetector platform demonstrated here was able to distinguish analyte DNA strands (20 base pairs) with mismatched nucleotide sequences (containing 1 and 2 base-pair mismatches) from complementary DNA strands. Based on these findings, GrO may serve as cheap and scalable alternative to realize rapid and point-of-care medical diagnostic solutions for screening of diseases related to single nucleotide polymorphisms.

Probing the local nature of excitons and plasmons in few-layer MoS2

Excitons and plasmons are the two most fundamental types of collective electronic excitations occurring in solids. Traditionally, they have been studied separately using bulk techniques that probe their average energetic structure over large spatial regions. However, as the dimensions of materials and devices continue to shrink, it becomes crucial to understand how these excitations depend on local variations in the crystal- and chemical structure on the atomic scale. Here, we use monochromated low-loss scanning-transmission-electron-microscopy electron-energy-loss spectroscopy, providing the best simultaneous energy and spatial resolution achieved to-date to unravel the full set of electronic excitations in few-layer MoS2 nanosheets over a wide energy range. Using first-principles, many-body calculations we confirm the excitonic nature of the peaks at ~ 2 and ~ 3 eV in the experimental electron-energy-loss spectrum and the plasmonic nature of higher energy-loss peaks. We also rationalise the non-trivial dependence of the electron-energy-loss spectrum on beam and sample geometry such as the number of atomic layers and distance to steps and edges. Moreover, we show that the excitonic features are dominated by the long wavelength (q = 0) components of the probing field, while the plasmonic features are sensitive to a much broader range of q-vectors, indicating a qualitative difference in the spatial character of the two types of collective excitations. Our work provides a template protocol for mapping the local nature of electronic excitations that open new possibilities for studying photo-absorption and energy transfer processes on a nanometer scale.

Comparison of wurtzite GaN/AlN and ZnO/MgO short‐period superlattices: Calculation of band gaps and built‐in electric field

The group‐III nitride and the group‐II oxide semiconductors have direct band gaps, which cover the ultraviolet to infrared energy range. In this work we calculate the band gaps and built‐in electric field of the polar wurtzite GaN/AlN and ZnO/MgO Short Period Superlattices (SPSLs). In many respects GaN and AlN are similar to ZnO and MgO, respectively, especially regarding the band gaps and the lattice parameters. To realize the wider band gap based materials the superlattices (SLs) with GaN and ZnO as quantum wells and AlN and MgO as quantum barriers, that is, GaN/AlN and ZnO/MgO, are created. We found similar evolution of the GaN/AlN and ZnO/MgO band gaps with varying number of atomic layers constituting these SPSLs. Band gap bowings and strength of the internal electric field existing in these two families of SPSLs differ significantly.

Material structure of two-/three-dimensional Si–C layers fabricated by hot-C+-ion implantation into Si-on-insulator substrate

We experimentally studied the material structures of two-/three-dimensional (2D/3D) silicon carbon layers Si1−YCY with Y ≤ 0.25 and 5 ≤ NL ≤ 162 [NL is the atomic layer number of Si1−YCY)] on buried oxide (BOX), which were fabricated by hot-C+-ion implantation into a (100) silicon-on-insulator (SOI) substrate before an oxidation process. A 2D Si layer was also fabricated as a reference. The C 1s spectrum obtained by X-ray photoemission spectroscopy shows that the implanted C atoms segregate at the oxide interface. Using a scanning transmission electron microscope and a high-resolution scanning transmission electron microscope to observe cross sections of Si0.75C0.25 layers, 2-nm-thick 3C-SiC layers were found be partially formed in the C segregation layer near the BOX interface. At Y > 0.1 and 5 ≤ NL ≤ 162, we observed very strong photoluminescence (PL) emission in the UV/visible regions from a 3C-SiC area and a Si1−YCY area in the C segregation layer, whereas a 2D Si emitted weak PL photons only at NL < 10. Thus, the silicon carbon technique is very promising for Si photonics and bandgap engineering in CMOS.

Geometric and electronic properties of ultrathin anatase TiO2(001) films.

Ultrathin anatase TiO2(001) films have recently been shown to exhibit many exotic properties, which are not observed in their thick counterpart. In this work, the dependence of the geometric and electronic properties of ultrathin anatase TiO2(001) films on the number of O-Ti-O trilayers is investigated on the basis of first-principles calculations. It is interesting to find that the lattice parameters, intertrilayer distances, electronic band gap and the position of the valence band edge for the films depend strongly on the number of trilayers, and they exhibit pronounced odd-even oscillations with the number of trilayers. Moreover, the convergence of geometric and electronic properties with the number of trilayers is rather slow, and not achieved even in films with 12 trilayers (∼2.7 nm). In addition, the adsorption state of the H2O molecule on the surface depends strongly on the thickness of the film. On the other hand, because the TiO2(001) surface is well described by a (001) film containing more than four trilayers, films with different lattice parameters and more than four trilayers may also be regarded as (001) surfaces that have been strained to different extents. Then, the above results from them also apply to the strained TiO2(001) surfaces. These results not only present new physics of the TiO2(001) films and surfaces, but also are helpful for understanding and modulating their performance in photocatalytic water splitting.

The slabs for the rutile TiO2 (110) surface**Project supported in part by the Major Program of National Science Foundation of China (No. 51090385) and the National Natural Science Foundation of China (No. 50974067).

Traditionally, we use a slab to mimic a surface and we constrain the slab to have the bulk-terminated 2D lattice constants. Here we propose a different model in which we impose no constraints, allowing all coordinates including the 2D slab itself to relax. We perform DFT calculations on both models. We find that the unconstrained slabs yield better agreement with experimental results and they have lower total energies. The optimized 2D lattice constants of the unconstrained slabs eventually converge to the attached bulk value. The total energy difference reveals that, with odd number trilayers, the unconstrained slab is much closer to the corresponding constrained slab. The surface energies both converge to the individual values with the number of atomic layers.

Raman scattering and anomalous Stokes-anti-Stokes ratio in MoTe2 atomic layers.

Stokes and anti-Stokes Raman scattering are performed on atomic layers of hexagonal molybdenum ditelluride (MoTe2), a prototypical transition metal dichalcogenide (TMDC) semiconductor. The data reveal all six types of zone center optical phonons, along with their corresponding Davydov splittings, which have been challenging to see in other TMDCs. We discover that the anti-Stokes Raman intensity of the low energy layer-breathing mode becomes more intense than the Stokes peak under certain experimental conditions, and find the effect to be tunable by excitation frequency and number of atomic layers. These observations are interpreted as a result of resonance effects arising from the C excitons in the vicinity of the Brillouin zone center in the photon-electron-phonon interaction process.

Optical response of silver clusters and their hollow shells from linear-response TDDFT

We present a study of the optical response of compact and hollow icosahedral clusters containing up to 868 silver atoms by means of time-dependent density functional theory. We have studied the dependence on size and morphology of both the sharp plasmonic resonance at 3–4 eV (originated mainly from sp-electrons), and the less studied broader feature appearing in the 6–7 eV range (interband transitions). An analysis of the effect of structural relaxations, as well as the choice of exchange correlation functional (local density versus generalised gradient approximations) both in the ground state and optical response calculations is also presented. We have further analysed the role of the different atom layers (surface versus inner layers) and the different orbital symmetries on the absorption cross-section for energies up to 8 eV. We have also studied the dependence on the number of atom layers in hollow structures. Shells formed by a single layer of atoms show a pronounced red shift of the main plasmon resonances that, however, rapidly converge to those of the compact structures as the number of layers is increased. The methods used to obtain these results are also carefully discussed. Our methodology is based on the use of localised basis (atomic orbitals, and atom-centered and dominant-product functions), which bring several computational advantages related to their relatively small size and the sparsity of the resulting matrices. Furthermore, the use of basis sets of atomic orbitals also allows the possibility of extending some of the standard population analysis tools (e.g. Mulliken population analysis) to the realm of optical excitations. Some examples of these analyses are described in the present work.

(Invited) Microwave Plasmas Applied for Synthesis of Free-Standing Carbon Nanostructures at Atmospheric Pressure Conditions

A microwave plasma-assisted method for synthesis of advanced carbon nanostructures is presented. The method is based on the injection of gas/liquid carbon precursors into a surface-wave sustained argon plasma environment, where the decomposition of molecules takes place. Gas-phase carbon atoms and molecules created in the "hot" plasma diffuse into colder zones of plasma reactor and aggregate into solid carbon nuclei. The main part of the solid carbon is gradually withdrawn from the "hot" plasma region in the outlet plasma stream where flowing carbon nanostructures assemble and grow. Selective synthesis of free-standing graphene sheets and diamond-like nanostructures was achieved. The engineering of the graphene sheets’ structural qualities and the control of the number of atomic layers per sheet is achieved via synergistic tailoring of the "hot"plasma environment and thermodynamic conditions in the post-discharge zone. The produced sheets (1-5 atomic layers) are stable and present much better qualities than those of graphene assembled via conventional methods, while being synthesized without the need of metal/crystal substrates. The synthesized structures have been analyzed by Raman spectroscopy, scanning electron microscopy, high-resolution transmission electron microscopy and X-ray photoelectron spectroscopy. It is worth noting that milligrams of high quality self-standing graphene sheets in a readily dispersive form can be obtained within a minute, through a single step process at atmospheric pressure conditions. The method is widely open for scale-up by using large-scale configurations of wave driven plasmas. Acknowledgements: This work was supported by Fundação para a Ciência e a Tecnologia, under 2015 Strategic Project.

Spin-state transition in antiferromagneticNi0.4Mn0.6films in Ni/NiMn/Ni trilayers on Cu(001)

The influence of the antiferromagnetic (AFM) spin structure of epitaxial ${\mathrm{Ni}}_{0.4}{\mathrm{Mn}}_{0.6}$ films on the magnetic properties of adjacent out-of-plane-magnetized ferromagnetic (FM) Ni layers in $\text{Ni}/{\mathrm{Ni}}_{0.4}{\mathrm{Mn}}_{0.6}/\text{Ni}$ trilayers on Cu(001) is investigated by magneto-optical Kerr-effect experiments. An AFM interlayer coupling between the two FM layers, which emerges below the AFM ordering temperature for an odd number of atomic layers of the antiferromagnet, suddenly disappears at a lower, AFM-thickness-dependent transition temperature. For even numbers of atomic layers of the antiferromagnet, a maximum of the coercivity is observed at a corresponding temperature. This result is interpreted as a transition of the AFM spin structure of the ${\mathrm{Ni}}_{0.4}{\mathrm{Mn}}_{0.6}$ layer that strongly affects the interlayer exchange coupling of the two FM layers by direct exchange through the AFM layer.

Effective photoconductivity of exfoliated black phosphorus for optoelectronic switching under 1.55 μm optical excitation

We present a microwave photoconductive switch based on exfoliated black phosphorus and strongly responding to a 1.55 μm optical excitation. According to its number of atomic layers, exfoliated black phosphorus presents unique properties for optoelectronic applications, like a tunable direct bandgap from 0.3 eV to 2 eV, strong mobilities, and strong conductivities. The switch shows a maximum ON/OFF ratio of 17 dB at 1 GHz, and 2.2 dB at 20 GHz under 1.55-μm laser excitation at 50 mW, never achieved with bidimensional materials.

Theoretical study on two-dimensional MoS2 piezoelectric nanogenerators

Recent experiments have demonstrated that nanogenerators fabricated using two-dimensional MoS2 flakes may find potential applications in electromechanical sensing, wearable technology, pervasive computing, and implanted devices. In the present study, we theoretically examined the effect of the number of atomic layers in MoS2 flakes on the nanogenerator output. Under a square-wave applied strain, MoS2 flakes with an even number of atomic layers did not exhibit a piezoelectric output, owing to the presence of a projected inversion symmetry. On the other hand, for MoS2 flakes with an odd number of layers, owing to the lack of inversion symmetry, piezoelectric output voltage and current were generated, and decreased with the increase of the number of layers. Furthermore, as MoS2 flakes were only a few atoms thick, the capacitance of the MoS2 nanogenerators was at least an order of magnitude smaller than that of the nanowire- and nanofilm-based nanogenerators, enabling the use of MoS2 nanogenerators in high-frequency applications. Our results explain the experimental observations and provide guidance on optimizing and designing two-dimensional nanogenerators.

Variations in Crystalline Structures and Electrical Properties of Single Crystalline Boron Nitride Nanosheets.

We report the studies of (1) the basic mechanism underlying the formation of defect-free, single crystalline boron nitride nanosheets (BNNSs) synthesized using pulsed laser plasma deposition (PLPD) technique, (2) the variation in the crystalline structure at the edges of the hexagonal boron nitride (h-BN) nanosheets, and (3) the basic electrical properties related to the BNNSs tunneling effect and electrical breakdown voltage. The nanoscale morphologies of BNNSs are characterized using scanning electron microscope (SEM) and high-resolution transmission electron microscope (HRTEM). The results show that each sample consisted of a number of transparent BNNSs that partially overlapped one another. Varying the deposition duration yielded different thicknesses of sample but did not affect the morphology, structure, and thickness of individual BNNSs pieces. Analysis of the SEM and HRTEM data revealed changes in the spatial period of the B3–N3 hexagonal structures and the interlayer distance at the edge of the BNNSs, which occurred due to the limited number of atomic layers and was confirmed further by x-ray diffraction (XRD) study. The experimental results clearly indicate that the values of the electrical conductivities of the super-thin BNNSs and the effect of temperature relied strongly on the direction of observation.

Thin-Shell Thickness of Two-Dimensional Materials

In applying the elastic shell models to monolayer or few-layer two-dimensional (2D) materials, an effective thickness has to be defined to capture their tensile and out-of-plane mechanical behaviors. This thin-shell thickness differs from the interlayer distance of their layer-by-layer assembly in the bulk and is directly related to the Föppl–von Karman number that characterizes the mechanism of nonlinear structural deformation. In this work, we assess such a definition for a wide spectrum of 2D crystals of current interest. Based on first-principles calculations, we report that the discrepancy between the thin-shell thickness and interlayer distance is weakened for 2D materials with lower tensile stiffness, higher bending stiffness, or more number of atomic layers. For multilayer assembly of 2D materials, the tensile and bending stiffness have different scaling relations with the number of layers, and the thin-shell thickness per layer approaches the interlayer distance as the number of layers increases. These findings lay the ground for constructing continuum models of 2D materials with both tensile and bending deformation.

Lateral Bending Vibration of Nanoscale Ultra-Thin Beams Using a Semi-Continuum Model

A semi-continuum dynamical model involving the relaxation effects is constructed to investigate the mechanical properties of the lateral bending vibration of an ultra-thin beam with a nanoscale thickness. The governing equation of vibration is derived based on the Hamilton principle. Three different kinds of end supporting conditions including fully clamped, simply supported and cantilevered nanoscale ultra-thin beams are considered and various numerical results are analyzed and discussed in detail. Some comparisons between the semi-continuum and classical continuum models are presented. It is concluded that the variational tends of natural frequencies depend on the values of relaxation coefficient. For different relaxation coefficients, the natural frequencies may decrease or increase with an increase in the number of atomic layers. Hence an explanation is proposed for some contradictions that the bending stiffness of the nanoscale beams should be enhanced or weakened in current non-local elasticity theory. Moreover, the relations between the Young's modulus and the number of atomic layers are developed and it is clearly seen that the small scale has a significant effect on the Young's modulus. When the number of atomic layers, or the thickness of the beam is sufficiently large, the small scale effects disappear and the value of the Young's modulus is just recovered to the corresponding classical bulk material, which also proves the validity of the semi-continuum dynamical model proposed in this study.

Fast and Patternable Synthesis of Graphene and Transition Metal Dichalcogenide Materials Via Laser Annealing on Insulating Substrates

The two-dimensional layered materials such as graphene and transition metal dichalcogenides (TMDs) have recently attracted much interest due to their astonishing properties under reduced dimensionality. Exceptional physical properties including ultra high mobility with high electrical conductivity for graphene and transition from indirect to direct bandgap showing excellent field effect behavior in TMD’s; open up a wide range of potential applications in optoelectronics, energy harvesting, Li-ion batteries and supercapacitors. However, the synthesis approach by CVD does not allow pattern and requires the use of high temperature processes restricting the use of substrates that can sustain such temperatures. Furthermore, the films have to be transferred to an additional substrate due to the damage caused to the original substrate used for synthesis. Therefore, further improvement of the synthesis methods is an important step to pursue. Here, we use a laser approach to induce simultaneously grow and pattern of graphene, WSe2 and MoSe2 directly on insulating substrates ready for device fabrication without the necessity of additional transfer processes. Raman and X-ray Photoelectron Spectroscopies supported by TEM observations to confirm the synthesis and crystallinity of the synthesized films. The process can be tuned to achieve a controllable number of atomic layers of the synthesized material. Moreover, this approach can be extended to the synthesis of other materials. This laser annealing assisted method develops a new approach that is fast, cheap and patternable giving a step forward on the technology development for practical applications.

Phase transition in ising magnetic superlattice nanowires: Molecular field theory approach

Within the framework of the molecular-field theory approach, (l,r) hexagonal magnetic superlattice nanowire model in which the l atomic layers of material a alternate with r atomic layers of material b is considered. These materials may be both ferromagnetic and antiferromagnetic. The transition temperature Tc for the nanowires is calculated as a function of inter-and intralayer exchange constant by the transfer matrix method. In particular case, the results are presented for the (1,1), (2,1), (2,2), (3,1), (4,1) and (4,2) ferromagnetic superlattice nanowires. It has been determined that critical temperature increases with the layer number in one elementary unit cell increases and it decrease when the difference of numbers of atomic layers, belonging to deferent materials, increases.

Ultrafast phonon dynamics of epitaxial atomic layers of Bi on Si(111)

Ultrathin bismuth (Bi) layers on $\text{Si(111)-7}\ifmmode\times\else\texttimes\fi{}7$ undergo a structural phase transformation with reducing the number of atomic layers at 3 bilayers (BL). We investigate the phonon dynamics of the Bi films close to the phase transformation by pump-probe reflectivity measurements. Coherent ${A}_{1g}$ and ${E}_{g}$ phonons at 3 and 2 THz are clearly observed for the Bi layers with thicknesses down to 3 BL, confirming their rhombohedral crystalline structure. The ${A}_{1g}$ frequency exhibits an abrupt redshift and splits into two components at 3 BL, which are attributed to the vertical motions of Bi atoms localized at the surface and subsurface bilayers. The ${E}_{g}$ frequency, by contrast, shows a gradual blueshift with reducing the thickness, possibly due to the lateral compressive stress at the Bi/Si interface. Below 3 BL, no coherent phonon signal is detected, in agreement with the phase transformation to the black-phosphoruslike structure. Our observations indicate that the vertical vibrations are significantly softened at 3 BL, but become almost as hard as those in the bulk crystal by adding another bilayer.

Simulation Study of Short-Channel Effect in MOSFET with Two-Dimensional Materials Channel

The short-channel effect (SCE) in a MOSFET with an atomically thin MoS2 channel was studied using a TCAD simulator. We derived the surface potential roll-up, drain-induced barrier lowering (DIBL), threshold voltage, and subthreshold swing (SS) as indexes of the SCE and analyzed their dependency on the channel thickness (number of atomic layers) and channel length. The minimum scalable channel length for a one-atomic-layer-thick MoS2 MOSFET was determined from the threshold voltage roll-off to be 7.6 nm. The one-layer-thick device showed a small DIBL of 87 mV/V at a 20 nm gate length. By using high-k gate insulator, an SS lower than 70 mV/dec is achievable in sub-10-nm-scale devices.