Layer-dependent Properties Research Articles

Structural, electronic and phononic properties of PtSe2: from monolayer to bulk

The layer dependent structural, electronic and vibrational properties of the 1T phase of two dimensional (2D) platinum diselenide are investigated by means of state-of-the-art first-principles calculations. The main findings of the study are: (i) monolayer platinum diselenide has a dynamically stable 2D octahedral structure with 1.66 eV indirect band gap, (ii) the semiconducting nature of 1T-PtSe2 monolayers remains unaffected even at high biaxial strains, (iii) top-to-top (AA) arrangement is found to be energetically the most favorable stacking of 1T-PtSe2 layers, (iv) the lattice constant (layer-layer distance) increases (decreases) with increasing number of layers, (v) while monolayer and bilayer 1T-PtSe2 are indirect semiconductors, bulk and few-layered 1T-PtSe2 are metals, (vi) Raman intensity and peak positions of the A1g and Eg modes are found to be highly dependent on the layer thickness of the material, hence; the number of layers of the material can be determined via Raman measurements.

Strongly enhanced exciton-phonon coupling in two-dimensional WSe2

The recently emerging laminar transition metal dichalcogenides provide an unprecedented platform for exploring fascinating layer-dependent properties. Determining the dependence of exciton-phonon coupling (EPC) on dimensionality would set a foundation for these exotic thickness-dependent phenomena. Here we report the observation of layer-dependent EPC between the ${A}_{1g}(\mathrm{\ensuremath{\Gamma}})$ phonon and A\ensuremath{'} exciton in $\mathrm{WS}{\mathrm{e}}_{2}$ down to the monolayer limit. Our results uncover that the strength of EPC increases dramatically with a descent of layer thickness. Compared with their bulk counterparts, the strength of EPC in monolayer $\mathrm{WS}{\mathrm{e}}_{2}$ is enhanced by nearly an order of magnitude. Furthermore, our work demonstrates that the giant EPC in the monolayer plays a prominent role in the exotic interlayer EPC of $\mathrm{WS}{\mathrm{e}}_{2}$/boron nitride heterostructures. The gigantic EPC in the two-dimensional limit provides a firm basis for understanding and manipulating the peculiar phenomena and novel optoelectronic applications based on atomically thin $\mathrm{WS}{\mathrm{e}}_{2}$.

CO adsorption on nanoislands: Ni on Au(111).

The adsorption behavior of CO on Ni islands grown on Au(111) was studied with a combination of temperature programmed desorption, Fourier transform infrared spectroscopy, and surface resistivity measurements. The Au(111) herringbone reconstruction provides a template for the growth of ordered Ni islands, with evidence for the presence of strain and Au atoms within the islands. The islands grow radially until θNi ≈ 0.3 ML, after which subsequent Ni atoms contribute primarily to a second layer. We study saturated CO adsorption at 227 K over a range of Ni island sizes and find layer-dependent adsorption properties. For single-layer islands at low Ni coverage, CO adsorbs primarily in the atop position and desorbs at lower temperatures than on pure Ni, with a saturation CO coverage of about 0.5 CO/Ni. As second layer Ni grows, saturated CO coverages on the Ni approach unity, with higher desorption temperatures, but still with primarily atop CO. Based on previous studies, we propose that in the first Ni layer, ligand effects from the Au substrate and possibly Au in the islands and strain due to the Ni/Au lattice mismatch affect the Ni-CO bonds. CO adsorption behavior on the two-layer islands is qualitatively explained by a decrease in Au nearest neighbors and the presence of a more expanded/corrugated structure.

Interband Transitions in Monolayer and Few-Layer WSe2 Probed Using Photoexcited Charge Collection Spectroscopy.

Transition-metal dichalcogenides are currently under rigorous investigation because of their distinct layer-dependent physical properties originating from the corresponding evolution of the band structure. Here, we report the highly resolved probing of layer-dependent band structure evolution for WSe2 using photoexcited charge collection spectroscopy (PECCS). Monolayer, few-layer, and multilayer WSe2 can be probed in top-gate field-effect transistor platforms, and their interband transitions are efficiently observed. Our theoretical calculations show a great coincidence with the PECCS results, proving that the indirect Γ-K and Γ-Λ transitions as well as the direct K-K transition are clearly resolved in multilayer WSe2 by PECCS.

Synthesis of Optically Uniform Single Layer WS2 for Tunable Photoluminescence

Two-dimensional transition metal dichalcogenides (2D TMDs) are promising materials for next-generation opto-electronic and nano-electronic devices due to their unique tunable bandgap as a function of the number of layers. Especially, single-layer tungsten disulfides (WS2) is a direct band gap semiconductor with a gap of 2.1eV featuring strong photoluminescence and large exciton binding energy. Although synthesis of MoS2 and their layer dependent properties have been studied rigorously, little attention has been paid to the formation of single-layer WS2 and its layer dependent properties. Here we report the scalable synthesis of uniform single-layer WS2 film by a two-step chemical vapor deposition (CVD) method followed by a laser thinning process. As-synthesized few-layer WS2 film shows 3.8 nm thickness considered to be 4 – 6 layers, and the post-laser irradiation enables systematic layer-by-layer thinning of the as-synthesized WS2 down to single-layer. The PL intensity increases six-fold, while the PL peak shifts from 1.92 eV to 1.97 eV during the laser thinning from few-layers to single-layer. We also find WS2 layer dependent intensity variation of the exciton complexes in PL emission. Both a neutral exciton and a trion increases with decreasing WS2 film thickness; however, the neutral exciton is predominant in single-layer WS2. The tunable optical properties by precise control of WS2 layers could empower a great deal of flexibility in designing atomically thin optoelectronic devices. Figure 1

Size and Substrate Effects Modulating Thermodynamics Properties of Hydrogenation in Pd/Au(111) Studied by Electrochemical PCT Isotherm

To study the thermodynamic properties of Pd nanofilm grown on the specific substrates is an appropriate and pivotal method for understanding the reaction mechanisms of Pd-H system. Here, thermodynamics of hydrogen in Pd/Au(111) nanofilms are investigated firstly by electrochemical pressure-composition-temperature (PCT) isotherm measurements followed by Van't Hoff plotting, through which the enthalpy (ΔH) and entropy (ΔS) are determined well. The results show layer dependent thermodynamic properties of them. Pd nanofilms with thickness lower than 53 monolayers are less affinity to hydrogen because of substrate and size effects. However, the hydrides formed on them are stable at room temperature with the stability lower than that of bulk Pd, which indicates Pd nanofilms with appropriate and dominated orientation surface can be the viable coats of the hydrogen storage materials for intaking and releasing hydrogen efficiently, and also enlarging the duration of them.

Study of the layer-dependent properties of MoS2 nanosheets with different crystal structures by DFT calculations

The main features of the electronic structure of MoS2 nanosheets are contributed by the intra-layer interaction, and the inter-layer interaction only induces slight perturbation. But the latter has an important influence on the electronic structure of MoS2 ultrathin nanosheets, especially the monolayer.

Electronic properties of hybrid monolayer-multilayer MoS2 nanostructured materials

The remarkable, layer-dependent properties of molybdenum disulphide (MoS2), such as an appropriately small and sizable bandgap or interplay between spin and the valley degrees of freedom, make it an attractive candidate for photodetectors, electrominescent devices, valleytronic devices, etc. Using nanostructuring to manipulate the size in lateral direction or number of layers of MoS2, we are opening a new playground for exploring and tuning properties of such systems. Here, we theoretically study the electronic properties of nanostructured MoS2 systems consisting of monolayer and multilayer MoS2 regions. In our analysis we consider hybrid systems in which monolayer region is surrounded by multilayer region and vice versa. We show how energy spectra and localization of carriers are influenced by the size and shape of the regions in lateral direction, number of MoS2 layers in the multilayer region, and the edge structure. Finally, we demonstrate how to control localization of carriers in these hybrid systems, which could make them appealing candidates for optoelectronic devices. Our findings are extracted from a tight-binding model that includes non-orthogonal sp3d5 orbitals, nearest-neighbor hopping matrix elements, and spin-orbit coupling.

Directional mechanical and thermal properties of single-layer black phosphorus by classical molecular dynamics

Black phosphorus (BP) has received attention due to its own higher carrier mobility and layer dependent electronic properties, such as direct band gap. Interestingly, the single layer black phosphorus (SLBP) has had large popularity in applications related to thermoelectric, optoelectronic, and electronic devices. Here, we investigate the phonon spectrum, thermal conductivities, and stress strain effects. Robust anisotropy was mainly observed in the thermal conductivities together with the alongside zigzag (ZZ) direction value, compared to the armchair (AC) directions. We also investigated the attitude of stress that was anisotropic in both directions, and the stress effects were two times greater across the ZZ path than those in the AC direction at a low temperature. We obtained a Young’s modulus of 63.77 and 20.74 GPa in the AC and ZZ directions, respectively, for a strain range of 0.01. These results had good agreement with first principle calculations. Our study here is useful and significant for the thermal tuning of phosphorus-based nanoelectronics and thermalelectric applications of phosphorus.

Synthesis of uniform single layer WS2 for tunable photoluminescence

Two-dimensional transition metal dichalcogenides (2D TMDs) have gained great interest due to their unique tunable bandgap as a function of the number of layers. Especially, single-layer tungsten disulfides (WS2) is a direct band gap semiconductor with a gap of 2.1 eV featuring strong photoluminescence and large exciton binding energy. Although synthesis of MoS2 and their layer dependent properties have been studied rigorously, little attention has been paid to the formation of single-layer WS2 and its layer dependent properties. Here we report the scalable synthesis of uniform single-layer WS2 film by a two-step chemical vapor deposition (CVD) method followed by a laser thinning process. The PL intensity increases six-fold, while the PL peak shifts from 1.92 eV to 1.97 eV during the laser thinning from few-layers to single-layer. We find from the analysis of exciton complexes that both a neutral exciton and a trion increases with decreasing WS2 film thickness; however, the neutral exciton is predominant in single-layer WS2. The binding energies of trion and biexciton for single-layer WS2 are experimentally characterized at 35 meV and 60 meV, respectively. The tunable optical properties by precise control of WS2 layers could empower a great deal of flexibility in designing atomically thin optoelectronic devices.

Electronic properties of bilayer phosphorene quantum dots in the presence of perpendicular electric and magnetic fields

Using the tight-binding approach, we investigate the electronic properties of bilayer phosphorene (BLP) quantum dots (QDs) in the presence of perpendicular electric and magnetic fields. Since BLP consists of two coupled phosphorene layers, it is of interest to examine the layer-dependent electronic properties of BLP QDs, such as the electronic distributions over the two layers and the so-produced layer-polarization features, and to see how these properties are affected by the magnetic field and the bias potential. We find that in the absence of a bias potential only edge states are layer-polarized while the bulk states are not, and the layer-polarization degree (LPD) of the unbiased edge states increases with increasing magnetic field. However, in the presence of a bias potential both the edge and bulk states are layer-polarized, and the LPD of the bulk (edge) states depends strongly (weakly) on the interplay of the bias potential and the interlayer coupling. At high magnetic fields, applying a bias potential renders the bulk electrons in a BLP QD to be mainly distributed over the top or bottom layer, resulting in layer-polarized bulk Landau levels (LLs). In the presence of a large bias potential that can drive a semiconductor-to-semimetal transition in BLP, these bulk LLs exhibit different magnetic-field dependences, i.e., the zeroth LLs exhibit a linear-like dependence on the magnetic field while the other LLs exhibit a square-root-like dependence.

Layer-number dependence of the magnetic properties of MnBi films

We present the effect of multilayer deposition on the magnetic properties of MnBi thin films. The multilayered MnBi films were fabricated with alternating deposition of Bi and Mn with N (N=2–10) layers (L), followed by in situ thermal annealing. As N increases, the even-layered and odd-layered films show a similar layer-number-dependence of the magnetic properties, such as an increasing behavior for Hc and a concave-up behavior for Mr and (BH)max. Higher magnetic properties were obtained at 9L and 10L. According to the electron microscopy analyses, different morphologies and elemental distributions were observed at different N (e.g., smooth-surfaced MnBi film with Bi islands at 2L and uneven-surfaced MnBi film at 10L). The different distributions of microstructure and chemical components are associated with the layer-dependent magnetic properties of the MnBi films.

Advances in Two‐Dimensional Layered Materials

Advances in Two‐Dimensional Layered Materials

Photocatalytic oxygen evolution from low-bandgap conjugated microporous polymer nanosheets: a combined first-principles calculation and experimental study.

Nanostructured semiconducting polymers have emerged as a very promising class of metal-free photocatalytic materials for solar water splitting. However, they generally exhibit low efficiency and lack the ability to utilize long-wavelength photons in a photocatalytic oxygen evolution reaction (OER). Here, based on first-principles calculations, we reveal that the two-dimensional (2D) aza-fused conjugated microporous polymer (aza-CMP) with a honeycomb network is a semiconductor with novel layer-dependent electronic properties. The bandgap of the as-synthesized aza-CMP nanosheets is measured to be 1.22 eV, suggesting that they can effectively boost light absorption in the visible and near infrared (NIR) region. More importantly, aza-CMP also possesses a valence band margin suitable for a photocatalytic OER. Taking advantage of the 2D layered nanostructure, we further show that the exfoliated ultrathin aza-CMP nanosheets can exhibit a three-fold enhancement in the photocatalytic OER. After deposition of a Co(OH)2 cocatalyst, the hybrid Co(OH)2/aza-CMP photocatalyst exhibits a markedly improved performance for photocatalytic O2 evolution. Furthermore, first-principles calculations reveal that the photocatalytic O2 evolution reaction is energetically feasible for aza-CMP nanosheets under visible light irradiation. Our findings reveal that nanostructured polymers hold great potential for photocatalytic applications with efficient solar energy utilization.

Layer-dependent properties ofSnS2andSnSe2two-dimensional materials

The layer dependent structural, electronic and vibrational properties of SnS2 and SnSe2 are investigated using first-principles density functional theory (DFT). The in-plane lattice constants, interlayer distances and binding energies are found to be layer-independent. Bulk SnS2 and SnSe2 are both indirect band gap semiconductors with Eg = 2.18 eV and 1.07 eV, respectively. Few-layer and monolayer 2D systems also possess an indirect band gap, which is increased to 2.41 eV and 1.69 eV for single layers of SnS2 and SnSe2. The effective mass theory of 2D excitons, which takes into account the combined effect of the anisotropy, non-local 2D screening and layer-dependent 3D screening, predicts strong excitonic effects. The binding energy of indirect excitons in monolayer samples, Ex~0.9 eV, is substantially reduced to Ex = 0.14 eV in bulk SnS2 and Ex = 0.09 eV in bulk SnSe2. The layer-dependent Raman spectra display a strong decrease of intensities of the Raman active A1g mode upon decreasing the number of layers down to a monolayer, by a factor of 7 in the case of SnS2 and a factor of 20 in the case of SnSe2 which can be used to identify number of layers in a 2D sample.

Electronic structure and properties of layered gallium telluride

Layer-dependent electronic structure and properties of gallium monochalcogenides, GaX where X=S, Se, Te, have been investigated using first-principles calculations based on various functionals, with a motivation to assess their use in photocatalytic water splitting. Since hydrogen evolution by water splitting using visible light provides a promising way for solar energy conversion, both theoretical and experimental studies have been carried out on the photochemical hydrogen evolution by GaTe. We also present the Raman spectra of GaTe examined by both theory and experiment.

Stacking-Dependent Interlayer Coupling in Trilayer MoS₂ with Broken Inversion Symmetry.

The stacking configuration in few-layer two-dimensional (2D) materials results in different structural symmetries and layer-to-layer interactions, and hence it provides a very useful parameter for tuning their electronic properties. For example, ABA-stacking trilayer graphene remains semimetallic similar to that of monolayer, while ABC-stacking is predicted to be a tunable band gap semiconductor under an external electric field. Such stacking dependence resulting from many-body interactions has recently been the focus of intense research activities. Here we demonstrate that few-layer MoS2 samples grown by chemical vapor deposition with different stacking configurations (AA, AB for bilayer; AAB, ABB, ABA, AAA for trilayer) exhibit distinct coupling phenomena in both photoluminescence and Raman spectra. By means of ultralow-frequency (ULF) Raman spectroscopy, we demonstrate that the evolution of interlayer interaction with various stacking configurations correlates strongly with layer-breathing mode (LBM) vibrations. Our ab initio calculations reveal that the layer-dependent properties arise from both the spin-orbit coupling (SOC) and interlayer coupling in different structural symmetries. Such detailed understanding provides useful guidance for future spintronics fabrication using various stacked few-layer MoS2 blocks.

Dry Electrochemical Etching of MoS2 Thin Films Using Plasma System

The graphene has been spot lighted as next generation 2D materials with excellent electrical, mechanical and optical properties for various fields of application. Recently, development of the wafer scale 2D thin film materials synthesis process leads to commercialization for the wearable devices. However, the zero band-gap of graphene has limitations for ‘off current’ realization, which attribute to difficult in utilizing for semiconductor devices. On the other hands, among the transition metal dichalcogenides (TMSs), molybdenum sulfide (MoS2) one can tune the band-gap depending on the number of layers from the 1.2 eV for bulk state to 1.8 eV for monolayer. It is reported that the layer dependent properties of MoS2 play important role in phototransistor application. Meanwhile, number of layers for MoS2 thin film can be controlled by the sulfurization of metal molybdenum (Mo). This process depends on pre-deposited Mo thickness and reaction with dissociated H2S gas during chemical vapor deposition (CVD). Also, in addition to the controlled growth methods, various etching techniques have been investigated as like the laser thinning method, thermal ablation and thermal annealing at 650 oC. These methods were able to etch the MoS2 thin film layers without changing the crystalline quality and surface roughness. However, requiring a high temperature process and a long process time were remain as challenge. Recently, though Ar+ plasma has been investigated for the controlled layer through electrochemical etching of patterned MoS2 thin films, this cause the limitation for physical damage on the surface due to the physical bombardment of Ar+ ion. Also, chemical etching of MoS2 thin film using XeF2 gas. However, the etch rate was not a linear function of etch time due to heat dissipation during the chemical reaction indicating the difficulties in the precise layers control. Therefore, the effective process for thickness control in 2D semiconductor thin film materials, such as MoS2, is needed for various flexible device application. In this study, A few layered MoS2 thin films were synthesized by plasma enhanced CVD (PECVD) and followed by dry electrochemical etching with the help of CF4 inductively coupled plasma (ICP). First the MoS2 thin films were synthesized in the PECVD system by Mo sulfurization on SiO2/Si wafers at 300 oC with optimized plasma conditions. Next, the synthesized six-layer MoS2 thin films were etched using a separate ICP etching system using CF4 plasma at room temperature with etching time variables. The results from Raman spectroscopy and atomic force microscopy (AFM) showed that, one layer of MoS2 thin film can be etched during 20 sec. exposure of CF4 plasma after an initial incubation time of 20 sec. The X-ray photoelectron spectroscopy (XPS) data reveals that there are damages and contaminations to MoS2 thin films during CF4 plasma exposure and these can be recovered effectively during 10 min. exposure of H2S plasma in PECVD. From these results we demonstrated that, the control on layer numbers in MoS2 (2D materials) is a facile and promising method for fabricating devices with the plasma processes.

Layer-dependent properties and noncollinear spin structure of epitaxial antiferromagnetic Mn films onCo/Cu(001)

The surface of expanded face-centered tetragonal antiferromagnetic Mn films of a few atomic monolayers thickness grown epitaxially on $\mathrm{Co}/\mathrm{Cu}(001)$ was investigated at room temperature by scanning tunneling microscopy and scanning tunneling spectroscopy using a ferromagnetic ring-shaped bulk iron probe. We show that the main contribution to the contrast modulation observed as a function of Mn thickness in differential conductance maps is not due to spin-polarized tunneling from a layerwise antiferromagnetic spin alignment. Instead, it is mainly of electronic origin resulting from layer-dependent electronic properties of the Mn film, probably related to different levels of intermixing with Co atoms. On the atomic scale, the Mn surface demonstrates a geometrical reconstruction with a $(12\ifmmode\times\else\texttimes\fi{}2)$ periodicity in two orthogonal domains on the fourfold symmetric substrate with an apparent surface corrugation of up to 0.3 \AA{}. Simultaneously recorded differential conductance maps show different textures in the two orthogonal domains, providing evidence for noncollinearity in the Mn surface spin structure.

Layer-dependent wall properties of abdominal aortic aneurysms: Experimental study and material characterization

Mechanical testing and in-depth characterization of the abdominal aortic aneurysm wall from fifteen patients undergoing open surgery was performed to establish the layer-dependent tissue properties that are non-available in the literature. Quantitative microscopic evaluation was performed to identify the spatial organization of collagen-fiber network. Among a number of candidate models, the four-fiber family (microstructure-motivated) model, especially that including dispersions of fiber angles about the main directions, was superior to the Fung- and Gasser-type models in the fitting quality allowed, though it presented a practical difficulty in parameter estimation, so that an analysis was conducted aiding the identification of a more specific diagonal- and circumferential-fiber family model for all three layers. The adventitia was stiffer and stronger than the other layers, owing to its increased collagen content, and its contribution to the response of the intact wall was augmented being under greater residual tension than the media, whereas the intima was under residual compression. All layers were stiffer circumferentially than longitudinally, due to preferential collagen arrangement along that axis. The histologically-guided material characterization of layered wall presented herein is expected to assist clinical decision, by developing reliable criteria to predict the rupture risk of abdominal aortic aneurysms, and optimize endovascular interventions.