Monatomic Layer Research Articles

Comprehensive study of optical contrast, reflectance, and Raman spectroscopy of multilayer graphene

Graphene research has developed quite rapidly partially because even a monatomic layer can be visualized with a conventional optical microscope. Although optical properties of multilayer graphene such as optical contrast, reflectance (R), and Raman scattering have been well studied, they are studied independently and the thickness dependence is limited to a rather thin region. In this paper, the evolution of optical properties by thickness from monolayer to multilayer graphene up to 107 nm thick is studied comprehensively. The empirically known change of color of multilayer graphene is confirmed from the R, G and B intensities extracted from the optical images. It is also found that, as far as R for visible light is concerned, multilayer graphene is not necessarily considered as a layered material, and the refractive index for monolayer graphene is applicable even for the thickest multilayer graphene flake in this study. On the other hand, the layered structure and Raman scattering at each layer are essential to reproduce the G-band intensity of Raman scattering (I(G)). Not only the multiple reflection but also the interference of scattered Raman light should be considered for I(G) of multilayer graphene thicker than ∼30 nm.

The Effect of a Monatomic Layer on a Surface on the Transition Radiation

The Effect of a Monatomic Layer on a Surface on the Transition Radiation

Verification of modulation mechanism of the interfacial dipole effect by changing the stacking sequence of monatomic layers in perovskite oxides

The magnitude of the dipole effect detected by the cutoff energy measurement was modulated for SrTiO3 (STO)/LaAlO3 (LAO)/SrRuO3 (SRO)/STO (001) subs. epitaxial stacks by ultrathin SrAlOx (SAO) insertion prior to LAO deposition. The difference in frictional forces on the LAO surfaces between the stacks with and without SAO insertion was clearly detected by lateral force microscopy (LFM), which indicated the change in dominating surface atomic layers. From the statistical analysis of LFM images, the SAO insertion was found to improve the lateral uniformity of the stacking sequence of charged monatomic layers in epitaxial LAO along the c-axis [(LaO)+ and (AlO2)−], taking account of the inevitable correlation between the surface-terminating atoms with the stacking sequence of the charged monatomic layers in epitaxial LAO. These results are consistently explainable with the proposed model that the interface dipole effect along the c-axis of the perovskite epitaxial stack is determined by the stacking sequence of charged monatomic layers of LAO.

Влияние осцилляций Фриделя на работу выхода нанопленок иттербия

The effect of standing waves of charge density (Friedel oscillations) generated by an interface of the ‘metallic ytterbium nanofilm  single-crystal silicon substrate’ type on the work function of ytterbium nanolayers has been studied. It is shown that in the range of nanofilm thicknesses from 0 to 8 monatomic layers, the work function has an oscillating character. This feature of the dependence of the work function on the nanofilm thickness is a consequence of the fact that the standing waves change nonmonotonically the power (momentum) of the electric double layer, which exists on the metal surface and affects the work function of the metal. This ultimately determines the oscillating nature of the dependence of the work function on the thickness of the nanofilms.

Effect of Friedel Oscillations on the Work Function of Ytterbium Nanofilms

The effect of standing waves of charge density (Friedel oscillations) generated by an interface of the metallic ytterbium nanofilm single-crystal silicon substrate type on the work function of ytterbium nanolayers has been studied. It is shown that in the range of nanofilm thicknesses from 0 to 8 monatomic layers, the work function has an oscillating character. This feature of the dependence of the work function on the nanofilm thickness is a consequence of the fact that the standing waves change nonmonotonically the power (momentum) of the electric double layer, which exists on the metal surface and affects the work function of the metal. This ultimately determines the oscillating nature of the dependence of the work function on the thickness of the nanofilms. Keywords: surface, nanofilm, work function, Friedel oscillations, electric double layer, ytterbium.

Topological defects and nanoholes in graphene oxide/hexagonal boron nitride heterostructures: stress buildup and accumulation.

The built-in distorted stress field of graphene (Gr) and its derivatives in defective state will induce local geometrical buckling due to the geometry of monatomic layer. The random distribution and types of functional groups (FGOs) and defects will have a significant impact on the stress accumulation and geometrical deformation of two-dimensional (2D) materials. By using molecular dynamics (MD), structure design and nonlinear mechanics theory, a new model (combining both planar 2D heterostructures and graphene oxide (GO)) was established to study geometrical effects, stress accumulation, bonding energies and mechanical properties of 2D interface (key point) at stress distortion field and accumulated stress field. The results show that grain boundaries (GBs), nanoholes and FGOs have different effects on the mechanical properties and out-of-plane deformation of 2D materials. By using Von-mises stresses and statistical mechanics, the geometrical effects, built-in distortion stress transfer and attenuation appeared in the each domain of 2D materials during the order-disorder transition processes. Moreover, there are two opposite aspects of stress accumulation, transmission, attenuation and geometrical effects of grain boundary (GBs), FGOs and nanoholes with distance. The ratio of strain energy (bond length and angle) is very sensitive to each domain of 2D materials. Finally, the 2D planar configuration gradually changes to a negative Gaussian surface, and the softening and weakening effects induced by GBs, nanoholes and FGOs are gradually enhanced. It is hoped that the current results can be used as a guide to adjust the geometry and stress accumulation of 2D materials in the new growth point.

First-principles study of Fe atom adsorbed biphenylene monolayer

Biphenylene monolayer is composed of four-, six- and eight-membered carbon rings and has a monatomic layer structure similar to graphene. It was synthesized in experiment recently and reported in <i>Science</i> in May 2021, which has attracted considerable attention in the research field of two-dimensional materials. By the density functional method of the first principle, we study the adsorption configuration of Fe atoms on biphenylene monolayer and analyze its electronic structure. The calculation of structural optimization, adsorption energy and molecular dynamics show that the biphenylene monolayer is a good matrix of Fe atoms. For Fe atoms, the hollow site in the four-membered ring of the biphenylene monolayer is the most stable adsorption site, and the adsorption energy can reach 1.56 eV. The calculation of charge transfer and density of states show that a stable bond can be formed between biphenylene monolayer and Fe atoms, and 0.73 electron is transferred from Fe atom to the neighbored carbon atom. After Fe atom being absorbed, biphenylene monolayer is magnetic, and the magnetic moment of Fe atom is about 1.81 <inline-formula><tex-math id="M2">\begin{document}${\mu}_{\mathrm{B}}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="3-20211631_M2.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="3-20211631_M2.png"/></alternatives></inline-formula> and points out of the plane. Compared with graphene, biphenylene monolayer adsorbs Fe atoms more stably, which provides a new platform for studying the electromagnetic, transport and catalytic properties of two-dimensional materials with adatoms.

Step-Edge Epitaxy for Borophene Growth on Insulators.

Borophene─a monatomic layer of boron atoms─stands out among two-dimensional (2D) materials, with its versatile properties tantalizing for physics exploration and next-generation devices. Yet its phases are all synthesized on and stay bound to metal substrates, hampering both characterization and use. Borophene growth on an inert insulator would allow postsynthesis exfoliation, but the weak adhesion to such a substrate results in a high 2D nucleation barrier, preventing clean borophene growth. This challenge can be circumvented in a strategy devised and demonstrated here with ab initio calculations. Naturally present 1D-defects, the step-edges on an h-BN substrate surface, enable boron epitaxial assembly, reduce the nucleation dimensionality, and lower the barrier by an order of magnitude (to 1.1 eV or less), yielding a v1/9 phase. Weak borophene adhesion to the insulator makes it readily accessible for comprehensive property tests or transfer into the device setting.

Monatomic Iodine Dielectric Layer for Multimodal Optical Spectroscopy of Dye Molecules on Metal Surfaces.

Fluorescence and Raman scattering spectroscopies have been used in various research fields such as chemistry, electrochemistry, and biochemistry because they can easily obtain detailed information about molecules at interfaces with visible light. In particular, multimodal fluorescence and Raman scattering spectroscopy have recently attracted significant attention, which enables us to distinguish chemical species and their electronic states that are important for expressing various functions. However, a special strategy is required to perform simultaneous measurements because the cross sections of fluorescence and Raman scattering differ by as much as ∼1014. In this study, we propose a method for the simultaneous measurement of dye molecules on a metal surface using a monatomic layer of iodine as the dielectric layer. The method is based on adequately quenching the photoexcited state of the molecules near the metal surface to weaken the fluorescence intensity and using the resonance effect to increase the Raman signal. We have validated this concept by experiments with insulating layers of different thicknesses and dye molecules of different chemical structures. The proposed multimodal strategy paves the way for various applications such as catalytic chemistry and electrochemistry, where the adsorption structure and electronic states of molecular species near the metal surface determine functionalities.

Germanene structure enhancement by adjacent insoluble domains of lead

Growth of Xene, referred to as the analog of graphene made of atoms other than carbon, with higher lattice quality and more intrinsic electronic structures, is an important challenge to the development of two-dimensional (2D) materials. We report an approach using Pb-atom deposition to striped-phase (SP) germanene grown on Ag(111) to trigger it to a quasifreestanding phase (QP). Using low-energy electron diffraction (LEED) and scanning tunneling microscopy (STM), we demonstrate that the 2D monatomic Pb layer, instead of being on top, grows side-by-side with QP germanene, not only driving the SP-to-QP transition but further enhancing the lattice order of QP as revealed by the dramatic evolution of LEED-spot shape and uniform STM-imaged array of moir\'e patterns. Originally oriented at 30\ifmmode^\circ\else\textdegree\fi{} to Ag(111), the QP germanene transits first to a narrow distribution of rotations then to two sharp domains at \ifmmode\pm\else\textpm\fi{}3\ifmmode^\circ\else\textdegree\fi{} from 30\ifmmode^\circ\else\textdegree\fi{}, while the monoatomic Pb domains twist from \ifmmode\pm\else\textpm\fi{}4.3\ifmmode^\circ\else\textdegree\fi{} to \ifmmode\pm\else\textpm\fi{}9.5\ifmmode^\circ\else\textdegree\fi{} with respect to Ag(111). The analysis of higher-order coincidence for both layers on Ag(111) is in complete agreement with LEED results and reveals a picture of two insoluble monatomic layers driving each other to a more optimal configuration. Moreover, the corresponding density of states measured by scanning tunneling spectroscopy for quality-enhanced QP germanene exhibits a clear V-shape distribution at the energy \ensuremath{\sim}0.4 eV above Fermi level, indicating the Fermi-Dirac cone, which agrees with our density functional theory calculation.

Monatomic Two-dimensional Layers; Modern Experimental Approaches for Structure, Properties, and Industrial Use

Monatomic Two-dimensional Layers; Modern Experimental Approaches for Structure, Properties, and Industrial Use

Двумерные материалы на основе элементов группы IVA: последние достижения в эпитаксиальных методах синтеза

Two-dimensional materials have become one of the central research topics of scientists around the world after the production of graphene - a monatomic layer of carbon. Currently, two-dimensional crystals are among the most promising materials for next-generation nanoelectronics and photonics. The exploration of the feasibility of 2D materials devices causes a deeper insight into the physical properties of these new materials and provides a starting point for the development of a number of important practical areas. Over the past few years, researchers have been attracting increased attention from graphene-like materials of group IVA elements, such as silicene (Si), germanene (Ge), stanene (Sn), and plumbene (Pb). Experimental production and study of the unique properties of two-dimensional monatomic layers of carbon, silicon, germanium, tin and lead on various substrates created the prerequisites for the development of new generation devices based on them. The wide possibilities for controlling their exotic electronic, magnetic and optical properties through the choice of the substrate, the design and geometry of the two-dimensional layer, as well as by controlling the magnitude of elastic stresses, have made them a dominating topic for studying in the field of nanotechnology and materials sciences. This paper reviews the latest advances in growing silicene, germanene, stanene, and plumbene using epitaxial methods. Growth technologies for creation of high-quality two-dimensional structures of large area required for promising instrumentation area are considered in more details.

Surface Phase Nucleation of Lead Monoatomic Layers on Si(111) Induced by Manganese Phthalocyanine Molecules

Hybrid interfaces where organic molecules are adsorbed on metallic substrates are very interesting to understand the fundamental interactions that might modify the chemical–physical properties of molecules or substrate. Here, we explore the adsorption of manganese phthalocyanines (MnPcs) on different structural phases of a Pb monatomic layer, namely, the √7 × √3-Pb and the striped incommensurate phase (SIC-Pb) phase, grown on Si(111). Surprisingly, the deposition of a minute amount of MnPc molecules (∼0.18 molecules/100 nm2) nucleates a macroscopic structural transition of the √7 × √3-Pb phase into the SIC-Pb phase. Our combined scanning tunneling microscopy, low-energy electron diffraction, and density functional theory study revealed that the mechanism behind this surface transformation is related to a strong and local molecule–substrate interaction. The structural phase transition is finally driven by the strained nature of the Pb phases and the energetic stability of the MnPc/SIC-Pb/Si(111) system with respect to the MnPc/√7 × √3-Pb/Si(111) one. The molecule–substrate interaction found in the present study is stronger than the one observed on Pb(111) bulk or thin films, highlighting the implication of the Pb/Si(111) interface in the interaction process. Hence, our results reveal that playing with the substrate dimensionality to tune the molecule–substrate coupling has strong impact on the electronic/magnetic properties of organic hybrid systems.

Ab-initio investigations on the energetic, opto-electronic and magnetic characteristics of alkali metal (AM) atom substituted monatomic AlN layer

Physical properties of AM atom substituted monolayer AlN are studied using first-principles calculations. During Li@Al substitution, AlN carries narrow/wide (0.3/2.95 eV) band gaps for spin up/down bands. The Na@Al substitution produces thin band semiconductor with (1.9/1.3 eV) energy gaps during spin up/down bands, respectively. Both K@Al and Na@N doping in AlN produced (0.5/0.4 eV) band gaps, respectively. The Li@N doped AlN is a half metal/wide band semiconductor for spin up/down bands, respectively. The K@N doped system displays (0.7/0.8 eV) energy gaps through spin up/down bands, respectively. Various optics parameters are improved in low energy range for all AM-doped AlN systems.

Ultra-Wide Spectral Bandwidth and Enhanced Absorption in a Metallic Compound Grating Covered by Graphene Monolayer

Graphene, a two-dimensional monatomic layer of carbon material, has demonstrated as a good candidate for applications of ultrafast photodetectors, transistors, transparent electrodes, and biosensors. Recently, many studies have shown that using metallic deep gratings could enhance the absorptance of graphene of 2.3% up to 80% in the near infrared region for applications in photon detection. This paper presents utilizing a nanograting structure, namely, a compound metallic grating could greatly enhance the absorptance of graphene up to 98% and widen its spectral bandwidth to 0.6 μm, which are greater than those of previous work. The study also showed that the absorptance spectrum is insensitive to angles of incidence. Furthermore, the proposed graphene-covered compound grating might bring a lot of benefits for graphene designs-based optical and optoelectronic devices.

First-principles study of stability, electronic structure and quantum capacitance of B-, N- and O-doped graphynes as supercapacitor electrodes

The structures, stability, electronic properties, and quantum capacitance (Cq) of α-, β-, and γ-graphyne monatomic layer doped with B, N, and O atoms, respectively, were studied using density functional theory. Two different doping sites (i.e. D1 and D2) were considered. Upon replacement of C atoms in the three graphynes with B and N atoms, the structure of graphynes was minimally distorted. This change was mainly manifested as a negligible adjustment of bond length around the doped atoms and lattice constant. However, with O atom doping, the structural distortion of graphynes was obvious in the majority of cases. The doping of these atoms significantly improved the electronic state of the original graphyne near the Fermi level, thereby improving graphyne Cq. Pristine graphynes with large pores and specific surface area exhibited better Cq performance than that of pure graphene. Cq of graphynes doped with B, N, and O showed significant advantage over that of doped graphene, especially that of α-, β-, and γ-graphyne with B doping at D1 and α-graphyne with O doping at D1. Interestingly, β-graphyne with O doping at D2 showed a considerably symmetrical Cq. Thus, element-doped graphynes have great potential as electrode materials for supercapacitors.

Improved understanding of the sulfidization mechanism in cerussite flotation: An XPS, ToF-SIMS and FESEM investigation

Sulfidization can greatly improve the flotation performance of lead oxide minerals. To provide deeper knowledge about this process, sulfidization of cerussite was investigated using X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectrometry (ToF-SIMS) and field emission electron microscopy (FESEM). XPS results clearly indicated a transformation of PbCO3 into PbS on the cerussite surfaces during sulfidization, which was consistent with ToF-SIMS results. Further, FESEM images showed that many nanoparticles grew on the cerussite at a high density, suggesting that the sulfidization product on the cerussite was not a monatomic layer but a solid polyatomic layer. The above results confirmed that PbS nanoparticles could grow on most of the surfaces of cerussite under flotation-related conditions, indicating that the sulfidized cerussite had a PbCO3/PbS core-shell structure. The formation of PbS nanoparticles on cerussite involved a heterogeneous nucleation step followed by growth of the nuclei. Based on these studies, a sulfidization process model was finally obtained.

Effective attenuation length dependence on photoelectron kinetic energy for gold from 1 keV to 10 keV: Role of island growth in overlayer experiments

We analyzed the experiments of Rubio-Zuazo and Castro [J. Electron Spectrosc. Relat. Phenom. 184 (2011) 384] who reported measurements of effective attenuation lengths (EALs) of gold for electron energies between 1 keV and 15 keV. They deposited thin films of gold on a copper substrate and measured the intensities of various Cu and Au photoelectron lines excited by synchrotron X-rays. Their derived EALs were based on the assumption that their films were of uniform thickness (i.e., layer-by-layer film growth). We considered two other forms of film growth: island formation (Volmer-Weber film growth) and island growth on a continuous monatomic wetting layer (Stranski-Krastanov film growth). We utilized the National Institute of Standards and Technology Database for the Simulation of Electron Spectra for Surface Analysis (SESSA) to determine the fractional Au island areas for each Au film thickness that would give the same intensity changes of Cu and Au photoelectron lines that were observed in the experiments. These results showed that growth of the Au film as islands provided a better description of the experimental results than uniform film growth. We could also distinguish Stranski-Krastanov film growth from Volmer-Weber growth and believe that the former is more likely.

Substrate-mediated umklapp scattering at the incommensurate interface of a monatomic alloy layer

Ultrathin Pb and Ge films deposited on Ag(111) surfaces have been investigated and compared. We found that at 1/3 ML, both films formed surface alloys, ${\mathrm{Ag}}_{2}\mathrm{Pb}$ and ${\mathrm{Ag}}_{2}\mathrm{Ge}$, with $\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}R{30}^{\ensuremath{\circ}}$ and $\frac{19}{20}\sqrt{3}\ifmmode\times\else\texttimes\fi{}\frac{19}{20}\sqrt{3}R{30}^{\ensuremath{\circ}}$ structures on Ag(111) but the surface electronic structures exhibit a most evident difference at the Ag(111) surface zone boundary ${\overline{M}}_{\mathrm{Ag}(111)}$, where the single band and the splitting ones were observed, respectively. Up to 1 ML, ${\mathrm{Ag}}_{2}\mathrm{Ge}$ subsequently develops into germanene with a striped phase and then a quasifreestanding phase, as previously reported [Lin et al., Phys. Rev. Mater. 2, 024003 (2018)], while ${\mathrm{Ag}}_{2}\mathrm{Pb}$ evolves to a dense Pb(111) phase that also reveals splitting bands at ${\overline{M}}_{\mathrm{Ag}(111)}$. We discover that the larger (smaller) atomic size of a Pb (Ge) atom with respect to an Ag atom causes the commensurate (incommensurate) interfaces and further demonstrate that the splitting bands of ${\mathrm{Ag}}_{2}\mathrm{Ge}$ surface alloy and 1-ML Pb film originated from the commonly incommensurate interface with Ag(111), which mediates umklapp scattering by inducing the mirror image of the pristine ${\mathrm{Ag}}_{2}\mathrm{Ge}$ and Pb(111) bands relative to ${\overline{M}}_{\mathrm{Ag}(111)}.$

Magnetic anisotropy of ferromagnetic metals in low-symmetry systems

We have constructed an analytic formula to treat the perpendicular magnetic anisotropy energy in ferromagnetic metals with low symmetry, such as C4V and C3V. We find that the anisotropy energy is proportional to a part of the expectation values of the orbital angular momentum and magnetic dipole operator. Although the result is similar to the model proposed by Laan (1998) [2], we have derived a concrete expression for the spin-flip virtual excitation process term, which can be dominant in atoms with small magnetic moments and/or small exchange splitting. Pt monatomic layer with proximity-induced spin polarization grown on Fe is an example of this. Other multilayer systems such as Co/Pd and Co/Ni, and bilayer systems such as Fe(CoB)/MgO can be discussed similarly. Moreover, the relation between perpendicular magnetic anisotropy energy and measurable physical parameters is discussed based on X-ray magnetic circular dichroism spectroscopy.