Substrate Step Edges Research Articles

Epitaxial Growth of Monolayer WS2 Single Crystals on Au(111) Toward Direct Surface-Enhanced Raman Spectroscopy Detection.

The epitaxial growth of wafer-scale two-dimensional (2D) semiconducting transition metal dichalcogenides (STMDCs) single crystals is the key premise for their applications in next-generation electronics. Despite significant advancements, some fundamental factors affecting the epitaxy growth have not been fully uncovered, e.g., interface coupling strength, adlayer-substrate lattice matching, substrate step-edge-guiding effects, etc. Herein, we develop a model system to tackle these issues concurrently, and realize the epitaxial growth of wafer-scale monolayer tungsten disulfide (WS2) single crystals on the Au(111) substrate. This epitaxial system is featured with good adlayer-substrate lattice matching, obvious step-edge-guiding effect for the unidirectionally aligned nucleation/growth, and relatively weaker interfacial interaction than that of monolayer MoS2/Au(111), as evidenced by the evolution of a uniform Moiré pattern and an intrinsic band gap, according to on-site scanning tunneling microscopy/spectroscopy (STM/STS) characterizations and density functional theory calculations. Intriguingly, the unidirectionally aligned monolayer WS2 domains along the Au(111) steps can behave as ultrasensitive templates for surface-enhanced Raman scattering detection of organic molecules, due to the obvious charge transfer occurred at substrate step edges. This work should hereby deepen our understanding of the epitaxy mechanism of 2D STMDCs on single-crystal substrates, and propel their wafer-scale production and applications in various cutting-edge fields.

Toward an understanding of the mechanism of mixed-salt-mediated CVD growth of MoSe2

The use of liquid precursors in chemical vapor deposition (CVD) techniques is advantageous for growing large-area, uniform two-dimensional (2D) transition-metal dichalcogenides (TMDs) compared to conventional methods using solid precursors. While various liquid precursors have been explored, recent studies highlight the use of mixed-salt precursors for growing uniform and wafer-scale TMDs. In this study, we propose a growth mechanism and present our findings on the epitaxial growth of MoSe2 domains as a function of annealing/growth time and H2 flow rate using Na2MoO4 and Na2SeO3 mixed-salt precursors. We confirm that the increase in the annealing time enhances the distribution of spin-coated precursors, leading to a rise in flake number density. On the other hand, prolonged growth time results in better-aligned MoSe2 flakes along the c-sapphire substrate step-edges. A significant finding is the dynamic diffusion of dendritic structures within large domains over the growth period, owing to the constant dissolution and recrystallization in the presence of residual liquid alloys. An increase in the H2 flow during CVD growth yields small, triangular domains aligned with the step edges, a result of the efficient reduction of precursor alloys and subsequent selenization. Our results provide an insight on achieving uniform and aligned morphology in CVD growth of 2D TMDs using liquid-phase precursors, a crucial step toward large-area fabrication.

Y‐Stabilized ZrO2 as a Promising Wafer Material for the Epitaxial Growth of Transition Metal Dichalcogenides

Y‐stabilized ZrO2 (YSZ) as a promising single‐crystal wafer material for the epitaxial growth of transition metal dichalcogenides applicable for both physical (PVD) and chemical vapor deposition (CVD) processes is used. MoS2 layers grown on YSZ (111) exhibit sixfold symmetry and in‐plane epitaxial relationship with the wafer of () MoS2 || () YSZ. The PVD‐grown submonolayer thin films show nucleation of MoS2 islands with a lateral size of up to 100 nm and a preferential alignment along the substrate step edges. The layers exhibit a strong photoluminescence yield as expected for the 2H‐phase of MoS2 in a single monolayer limit. The CVD‐grown samples are composed of triangular islands of several micrometers in size in the presence of antiparallel domains. The results represent a promising route toward fabrication of wafer‐scale single‐crystalline transition metal dichalcogenide layers with a tunable layer thickness on commercially available wafers.

The stable interfaces between various edges of hBN and step edges of Cu surface in hBN epitaxial growth: a comprehensive theoretical exploration

High-index Cu surfaces were broadly shown to be substrates capable for templating the epitaxial growth of uniformly aligned hexagonal boron nitride (hBN) islands whereas the mechanism of hBN growth on high-index Cu surfaces is still missing. Since hBN nucleation prefers step edges on a high-index Cu surface, the understanding of the interfaces between the hBN edges and the step edges of Cu substrates is critical for revealing the mechanism of hBN epitaxial growth on high-index Cu surfaces. Our extensive theoretical study reveals that both types of zigzag edges and armchair edge tend to retain their pristine structures on a Cu surface due to the effective passivation of the dangling bonds of hBN edges. This study paves a way to explore the growth kinetics of hBN on high-index Cu surfaces and also sheds light on the growth mechanisms of various two-dimensional materials on active metal substrates.

Nucleation, morphology, and structure of sub‐nm thin ceria islands on Rh(111)

The early stages of ceria growth on Rh(111) at high temperature have been investigated by low‐energy electron microscopy and photoemission electron microscopy. Ceria was deposited by reactive Ce deposition at substrate temperatures between 700°C and 900°C in an oxygen ambient of 5 × 10−7 Torr. At 700°C, we observe a high nucleation density of 100‐nm‐sized islands. With elevated temperature, the average island size increases, and the nucleation density decreases. Triangularly shaped islands nucleate preferentially at step edges, with seemingly abrupt interfaces between Ce and Rh. At 900°C, the island edges are still straight, but during growth the islands lose their triangular form. Instead, growth along the substrate step edges becomes favorable, leading to a maze‐like morphology. Atomic force microscopy reveals islands of 0.3 to 0.6‐nm height, consistent with ceria islands formed by one or two trilayers (O―Ce―O) of ceria. Moreover, the second layer of the islands is also triangularly shaped, with lateral dimensions of 50 nm and similar step heights. IV‐LEEM analysis leads to the conclusion that the rhodium surface is covered by a layer of reduced cerium oxide, which is partially overgrown by smaller islands of CeO2.

Realizing Large-Scale, Electronic-Grade Two-Dimensional Semiconductors.

Atomically thin transition metal dichalcogenides (TMDs) are of interest for next-generation electronics and optoelectronics. Here, we demonstrate device-ready synthetic tungsten diselenide (WSe2) via metal-organic chemical vapor deposition and provide key insights into the phenomena that control the properties of large-area, epitaxial TMDs. When epitaxy is achieved, the sapphire surface reconstructs, leading to strong 2D/3D (i.e., TMD/substrate) interactions that impact carrier transport. Furthermore, we demonstrate that substrate step edges are a major source of carrier doping and scattering. Even with 2D/3D coupling, transistors utilizing transfer-free epitaxial WSe2/sapphire exhibit ambipolar behavior with excellent on/off ratios (∼107), high current density (1-10 μA·μm-1), and good field-effect transistor mobility (∼30 cm2·V-1·s-1) at room temperature. This work establishes that realization of electronic-grade epitaxial TMDs must consider the impact of the TMD precursors, substrate, and the 2D/3D interface as leading factors in electronic performance.

Reaction kinetics of ultrathin NaCl films on Ag(001) upon electron irradiation

We report on an electron-induced modification of alkali halides in the ultrathin film regime. The reaction kinetics and products of the modifications are investigated in the case of NaCl films grown on Ag(001). Their structural and chemical modification upon irradiation with electrons of energy 52--60 eV and 3 keV is studied using low-energy electron diffraction (LEED) and Auger electron spectroscopy (AES), respectively. The irradiation effects on the film geometry and thickness (ranging from between two and five atomic layers) are examined using scanning tunneling microscopy (STM). We observe that Cl depletion follows different reaction kinetics, as compared to previous studies on NaCl thick films and bulk crystals. Na atoms produced from NaCl dissociation diffuse to bare areas of the Ag(001) surface, where they form Na-Ag superstructures that are known for the Na/Ag(001) system. The modification of the film is shown to proceed through two processes, which are interpreted as a fast disordering of the film with removal of NaCl from the island edges and a slow decrease of the structural order in the NaCl with formation of holes due to Cl depletion. The kinetics of the Na-Ag superstructure growth is explained by the limited diffusion on the irradiated surface, due to aggregation of disordered NaCl molecules at the substrate step edges.

Imaging initial formation processes of nanobubbles at the graphite–water interface through high-speed atomic force microscopy

The initial formation process of nanobubbles at solid–water interfaces remains unclear because of the limitations of current imaging techniques. To directly observe the formation process, an astigmatic high-speed atomic force microscope (AFM) was modified to enable imaging in the liquid environment. By using a customized cantilever holder, the resonance of small cantilevers was effectively enhanced in water. The proposed high-speed imaging technique yielded highly dynamic quasi-two-dimensional (2D) gas structures (thickness: 20–30 nm) initially at the graphite–water interface. The 2D structures were laterally mobile mainly within certain areas, but occasionally a gas structure might extensively migrate and settle in a new area. The 2D structures were often confined by substrate step edges in one lateral dimension. Eventually, all quasi-2D gas structures were transformed into cap-shaped nanobubbles of higher heights and reduced lateral dimensions. These nanobubbles were immobile and remained stable under continuous AFM imaging. This study demonstrated that nanobubbles could be stably imaged at a scan rate of 100 lines per second (640 μm/s).

Influence of support morphology on the bonding of molecules to nanoparticles

Supported metal nanoparticles form the basis of heterogeneous catalysts. Above a certain nanoparticle size, it is generally assumed that adsorbates bond in an identical fashion as on a semiinfinite crystal. This assumption has allowed the database on metal single crystals accumulated over the past 40 years to be used to model heterogeneous catalysts. Using a surface science approach to CO adsorption on supported Pd nanoparticles, we show that this assumption may be flawed. Near-edge X-ray absorption fine structure measurements, isolated to one nanoparticle, show that CO bonds upright on the nanoparticle top facets as expected from single-crystal data. However, the CO lateral registry differs from the single crystal. Our calculations indicate that this is caused by the strain on the nanoparticle, induced by carpet growth across the substrate step edges. This strain also weakens the CO-metal bond, which will reduce the energy barrier for catalytic reactions, including CO oxidation.

Defect-Tolerant Aligned Dipoles within Two-Dimensional Plastic Lattices.

Carboranethiol molecules self-assemble into upright molecular monolayers on Au{111} with aligned dipoles in two dimensions. The positions and offsets of each molecule's geometric apex and local dipole moment are measured and correlated with sub-Ångström precision. Juxtaposing simultaneously acquired images, we observe monodirectional offsets between the molecular apexes and dipole extrema. We determine dipole orientations using efficient new image analysis techniques and find aligned dipoles to be highly defect tolerant, crossing molecular domain boundaries and substrate step edges. The alignment observed, consistent with Monte Carlo simulations, forms through favorable intermolecular dipole-dipole interactions.

Bilayer-induced asymmetric quantum Hall effect in epitaxial graphene

The transport properties of epitaxial graphene on SiC(0001) at quantizing magnetic fields are investigated. Devices patterned perpendicularly to SiC terraces clearly exhibit bilayer inclusions distributed along the substrate step edges. We show that the transport properties in the quantum Hall regime are heavily affected by the presence of bilayer inclusions, and observe a significant departure from the conventional quantum Hall characteristics. In particular, we observe anomalous values of the quantized resistance and a peculiar asymmetry with magnetic field which was not observed before for graphene on SiC. A quantitative model involving enhanced inter-channel scattering mediated by the presence of bilayer inclusions is presented that successfully explains the observed symmetry properties.

Cobalt intercalation at the graphene/iridium(111) interface: Influence of rotational domains, wrinkles, and atomic steps

Using low-energy electron microscopy, we study Co intercalation under graphene grown on Ir(111). Depending on the rotational domain of graphene on which it is deposited, Co is found intercalated at different locations. While intercalated Co is observed preferentially at the substrate step edges below certain rotational domains, it is mostly found close to wrinkles below other domains. These results indicate that curved regions (near substrate atomic steps and wrinkles) of the graphene sheet facilitate Co intercalation and suggest that the strength of the graphene/Ir interaction determines which pathway is energetically more favorable.

Electrical Nanocharacterization of Epitaxial Graphene/Silicon Carbide Schottky Contacts

Epitaxial graphene fabricated by thermal decomposition of the Si-face of silicon carbide (SiC) forms a defined interface to the SiC substrate. As-grown monolayer graphene with buffer layer establishes an ohmic interface even to low-doped (e. g. [N] ≈ 1015 cm-3) SiC, and a specific contact resistance as low as ρC = 5.9×10-6 Ωcm2 can be achieved on highly n-doped SiC layers. After hydrogen intercalation of monolayer graphene, the so-called quasi-freestanding graphene forms a Schottky contact to n-type SiC with a Schottky barrier height of 1.5 eV as determined from C-V analysis and core level photoelectron spectroscopy (XPS). This value, however, strongly deviates from the respective value of less than 1 eV determined from I-V measurements. It was found from conductive atomic force microscopy (C-AFM) that the Schottky barrier is locally lowered on other crystal facets located at substrate step edges. For very small Schottky contacts, the barrier height extracted from I-V curves approaches the value of 1.5 eV from C-V and XPS.

The correlation of epitaxial graphene properties and morphology of SiC (0001)

The electronic properties of epitaxial graphene (EG) on SiC (0001) depend sensitively on the surface morphology of SiC substrate. Here, 2–3 layers of graphene were grown on on-axis 6H-SiC with different step densities realized through controlling growth temperature and ambient pressure. We show that epitaxial graphene on SiC (0001) with low step density and straight step edge possesses fewer point defects laying mostly on step edges and higher carrier mobility. A relationship between step density and EG mobility is established. The linear scan of Raman spectra combined with the atomic force microscopy morphology images revealed that the Raman fingerprint peaks are nearly the same on terraces, but shift significantly while cross step edges, suggesting the graphene is not homogeneous in strain and carrier concentration over terraces and step edges of substrates. Thus, control morphology of epitaxial graphene on SiC (0001) is a simple and effective method to pursue optimal route for high quality graphene and will be helpful to prepare wafer sized graphene for device applications.

Friction and atomic-layer-scale wear of graphitic lubricants on SiC(0001) in dry sliding

Sliding friction experiments on graphene grown on SiC(0001) have been performed using a combination of a microtribometer with an atomic force microscope (AFM) allowing for the investigation of atomic-scale wear. The graphene layer delaminates within 10 sliding cycles starting from substrate step edges. After run in, friction is dominated by the interaction between a changing configuration of asperities at the probe sphere and a graphitic interface layer terminating the SiC substrate. Friction varies unpredictably due to changes in the contact configuration. However, the linear relation between friction and contact area can be confirmed and a shear strength as low as a few MPa is found for the contact between ruby and the graphitic layer on SiC, which remains intact under continuous sliding.

Layer-by-layer growth of sodium chloride overlayers on an Fe(001)-p(1 × 1)O surface

Ultra-thin NaCl films epitaxially grown on an Fe(001)-p(1 × 1)O surface have been investigated in ultra-high vacuum by non-contact atomic force microscopy and low energy electron diffraction. It has been found that at temperatures below 145 °C NaCl initially grows as monoatomic thick islands on substrate terraces, while at temperatures above 175 °C biatomic thick islands are also formed at substrate step edges. Both types of islands have the same Fe(001)–O[100] ∥ NaCl(001)[110] orientation, leading to a (4 × 4) superstructure, where the NaCl unit cell is oriented at 45° with respect to the substrate. Interestingly, no c(2 × 2) superstructure with the NaCl unit cell oriented at 0° has been observed. The oxygen on the iron surface promotes layer-by-layer growth, resulting in atomically flat films with 40–60 nm wide terraces at coverages ranging from 0.75 to 12 ML. Such NaCl films are of much higher quality than MgO films grown on Fe(001) and Fe(001)-p(1 × 1)O surfaces and represent a unique epitaxial system of an alkali halide on a pure metallic substrate. The reduced number of defects and the layer-by-layer mode of growth make this system very attractive for applications where an atomically defined tunnel barrier is required to control the properties of a device.

Growth and ordering of Ni(II) diphenylporphyrin monolayers on Ag(111) and Ag/Si(111) studied by STM and LEED

The room temperature self-assembly and ordering of (5,15-diphenylporphyrinato)nickel(II) (NiDPP) on the Ag(111) and Ag/Si(111)-(√3 × √3)R30° surfaces have been investigated using scanning tunnelling microscopy and low-energy electron diffraction. The self-assembled structures and lattice parameters of the NiDPP monolayer are shown to be extremely dependent on the reactivity of the substrate, and probable molecular binding sites are proposed. The NiDPP overlayer on Ag(111) grows from the substrate step edges, which results in a single-domain structure. This close-packed structure has an oblique unit cell and consists of molecular rows. The molecules in adjacent rows are rotated by approximately 17° with respect to each other. In turn, the NiDPP molecules form three equivalent domains on the Ag/Si(111)-(√3 × √3)R30° surface, which follow the three-fold symmetry of the substrate. The molecules adopt one of three equivalent orientations on the surface, acting as nucleation sites for these domains, due to the stronger molecule–substrate interaction compared to the case of the Ag(111). The results are explained in terms of the substrate reactivity and the lattice mismatch between the substrate and the molecular overlayer.

High Mobility Epitaxial Graphene for Carbon-based Nanoelectronics

Epitaxial graphene has demonstrated a great potential for carbon-based electronic. Multiple nanoscale devices can be patterned on a scalable platform with high electronic mobility, large conductivity, and conductance modulation by an electrostatic gate. High quality seamless epitaxial graphene layers are grown on the entire surface of SiC substrates by thermal decomposition of the SiC crystal. For multilayer epitaxial graphene grown on the 4H-SiC (000-1) surface, transport and spectroscopy measurements demonstrate an effective decoupling of the adjacent graphene layers due to a non-graphitic ordered rotational stacking. Large switching ratios require a band gap but graphene is a gapless semimetal. Transport gaps have been demonstrated in chemical functionalized graphene and in narrow ribbons. But patterning techniques severely degrade graphene. Transport data on narrow graphene ribbons directly grown on silicon carbide substrate step edges at high temperature indicate reduced edge scattering and open the way to non diffusive electronics.

Tailoring magnetic anisotropy in epitaxial half metallic La0.7Sr0.3MnO3 thin films

We present a detailed study on the magnetic properties, including anisotropy, reversal fields, and magnetization reversal processes, of well characterized half-metallic epitaxial La0.7Sr0.3MnO3 (LSMO) thin films grown onto SrTiO3 (STO) substrates with three different surface orientations, i.e., (001), (110), and (11−8). The latter shows step edges oriented parallel to the [110] (in-plane) crystallographic direction. Room temperature high resolution vectorial Kerr magnetometry measurements have been performed at different applied magnetic field directions in the whole angular range. In general, the magnetic properties of the LSMO films can be interpreted with just the uniaxial term, with the anisotropy axis given by the film morphology, whereas the strength of this anisotropy depends on both structure and film thickness. In particular, LSMO films grown on nominally flat (110)-oriented STO substrates presents a well defined uniaxial anisotropy originated from the existence of elongated in-plane [001]-oriented structures, whereas LSMO films grown on nominally flat (001)-oriented STO substrates show a weak uniaxial magnetic anisotropy, with the easy axis direction aligned parallel to residual substrate step edges. Elongated structures are also found for LSMO films grown on vicinal STO(001) substrates. These films present a well-defined uniaxial magnetic anisotropy, with the easy axis lying along the step edges, and its strength increases with the LSMO thickness. It is remarkable that this step-induced uniaxial anisotropy has been found for LSMO films up to 120 nm thickness. Our results are promising for engineering novel half-metallic magnetic devices that exploit tailored magnetic anisotropy.

Conductance anisotropy and linear magnetoresistance inLa2 − xSrxCuO4 thin films

We have performed a detailed study of conductance anisotropy and magnetoresistance (MR) ofLa2 − xSrxCuO4 (LSCO)thin films (0.10 < x < 0.25). These two observables are promising for the detection of stripes. Subtle features ofthe conductance anisotropy are revealed by measuring the transverse resistanceRxy in zero magnetic field. It is demonstrated that the sign ofRxy depends on the orientation of the LSCO Hall bar with respect to the terrace structure ofthe substrate. Unit-cell-high substrate step edges must therefore be a dominant nucleationsource for antiphase boundaries during film growth. We show that the measurement ofRxy is sensitive enough to detect the cubic–tetragonal phase transition of theSrTiO3(100) (STO) substrate at 105 K. The MR of LSCO thin films shows for0.10 < x < 0.25 a non-monotonic temperature dependence, resulting from the onset of a linear term in theMR above 90 K. We show that the linear MR scales with the Hall resistivity as , with the constant of proportionality independent of temperature. Such scaling suggeststhat the linear MR originates from current distortions induced by structural or electronicinhomogeneities. The possible role of stripes for both the MR and the conductanceanisotropy is discussed throughout the paper.