First-Principles Study of Electronic, Optical, and Magnetic Properties of Fe-, Co-, and Ni-Doped MoS2 Monolayer

Band structure and spin–orbit coupling engineering in transition‐metal dichalcogenides

Borophene, a two-dimensional monolayer of boron atoms, was recently synthesized experimentally and was shown to exhibit polymorphism. In its closed-packed triangular form, borophene is expected to exhibit anisotropic metallic character with relatively high electron velocities. At the same time, very low optical conductivities in the infrared-visible light region were predicted. Based on its promising electronic transport properties and its high transparency, borophene could become a genuine lego piece in the 2D materials assembling game known as the van der Waals heterocrystal approach. However, borophene is naturally degraded in ambient conditions and it is therefore important to assess the mechanisms and the effects of oxidation on borophene monolayers. Optical and electronic properties of pristine and oxidized borophene are here investigated by first-principles approaches. The transparent and conductive properties of borophene are elucidated by analyzing the electronic structure and its interplay with light. Optical response of borophene is found to be strongly affected by oxidation, suggesting that optical measurements can serve as an efficient probe for borophene surface contamination.

First-principles study of antimony-doped monolayer molybdenum disulfide: Electronic structure and optical properties

Monolayer transition metal dichalcogenides (TMDCs) are promising two-dimensional (2D) semiconductors for application in optoelectronics. Their optical properties are dominated by two series of photo-excited exciton states—A (XA) and B (XB)1,2—that are derived from direct interband transitions near the band extrema. These exciton states have large binding energies and strong optical absorption3–6, and form an ideal system to investigate many-body effects in low dimensions. Because spin–orbit coupling causes a large splitting between bands of opposite spins, XA and XB are usually treated as spin-polarized Ising excitons, each arising from interactions within a specific set of states induced by interband transitions between pairs of either spin-up or spin-down bands (TA or TB). Here, by using monolayer MoS2 as a prototypical system and solving the first-principles Bethe–Salpeter equations, we demonstrate a strong intravalley exchange interaction between TA and TB, indicating that XA and XB are mixed states instead of pure Ising excitons. Using 2D electronic spectroscopy, we observe that an optical excitation of the lower-energy TA induces a population of the higher-energy TB, manifesting the intravalley exchange interaction. This work elucidates the dynamics of exciton formation in monolayer TMDCs, and sheds light on many-body effects in 2D materials. Two-dimensional electronic spectroscopy experiments and first-principles many-electron calculations demonstrate the quantum mixing of different exciton states in monolayer MoS2. This reveals the many-body effects and dynamics of exciton formation in 2D materials.

Electronic and optical properties of pristine and Li/Na/K/Mg/Ca decorated net-Y: First-principles calculations

Body: Semiconducting two-dimensional (2D) materials possess intriguing optoelectronic properties owing to the unique layered confined structure, such as strong light-matter interaction and large exciton binding energies. Especially, their excitonic binding energies far exceeding thermal energy will allow for room-temperature optoelectronic applications, such as excitonic computing applications and optical interconnects. It is therefore of significant importance to engineer excitonic properties and dynamics. The most widely used approaches to modulate the semiconducting 2D materials' excitonic dynamics include modifying the dielectric environment,[1] applying strain,[2] and gate-tuning.[3] In this study, we study modulation of exciton dynamics in monolayer MoS2 via dielectric Coulomb engineering and local strain by correlating photoluminescence (PL) mapping and scanning probe microscopy characterization. A mixed-dimensional heterojunction (MDHJ) was fabricated consisting of monolayer MoS2, sub-μm 0-dimensional P(VDF-TrFE) copolymer islands, and hBN. An optical microscope image, AFM topography map, and side view schematic of the MDJS are showing in Figures 1a, 1b, and 1c, respectively. Photoluminescence (PL) maps were obtained; A and B exciton emissions were fitted to Gaussians for parameter extraction (Figure 2a and 2b) and correlated with the topography data (Figure 2c). A clear blueshift of A exciton emission energy was observed on 1L-MoS2 on P(VDF-TrFE) islands with respect to 1L-MoS2 on hBN (Figure 2b). MoS2 on the polymer islands (red scatter plot; draped over polymer islands) showed a noticeable blueshift of ~3 meV compared to that of planar MoS2 on hBN (blue scatter plot) (Figure 2c). The bandgap increase can be attributed to the band structure renormalization and binding energy decrease due to the strong dielectric screening of the P(VDF-TrFE) substrate (εr ≈ 13).[1] Effects of dielectric environment and local strain on excitonic dynamics were examined by analyzing the A and B exciton intensity ratio; a high A-to-B intensity ratio is a sign of low nonradiative recombination.[4] Regions of increased A/B ratio (up to 2x) correlate well with the P(VDF-TrFE) islands locations (Figure 3a). Suppressed B exciton from the MoS2/P(VDF-TrFE) region contributes to the majority of the A/B ratio enhancement. Moreover, the B intensity showed a negative correlation with height, suggesting that large substrate screening combined with higher strain from the morphology enhances the intravalley scattering from the B exciton state to the A exciton state (Figure 3b). The A and B exciton energy difference map (Figure 3c) demonstrates that in contact with P(VDF-TrFE), the spin-orbit coupling-induced valence band splitting reduces, which may lead to an increased intravalley scattering rate. The correlation of PL and topography map revealed that the substrate dielectric screening and local strain affect the exciton dynamics of monolayer MoS2. [1] A. Raja et al., "Coulomb engineering of the bandgap and excitons in two-dimensional materials," Nat. Commun., vol. 8, no. May, p. 15251, May 2017. [2] A. Castellanos-Gomez et al., "Local Strain Engineering in Atomically Thin MoS 2," Nano Lett., vol. 13, no. 11, pp. 5361–5366, Nov. 2013. [3] Z. Qiu et al., "Giant gate-tunable bandgap renormalization and excitonic effects in a 2D semiconductor," Sci. Adv., vol. 5, no. 7, p. eaaw2347, Jul. 2019. [4] K. M. McCreary, A. T. Hanbicki, S. V. Sivaram, and B. T. Jonker, "A- and B-exciton photoluminescence intensity ratio as a measure of sample quality for transition metal dichalcogenide monolayers," APL Mater., vol. 6, no. 11, p. 111106, Nov. 2018.

First-principles investigations of metal (V, Nb, Ta)-doped monolayer MoS2: Structural stability, electronic properties and adsorption of gas molecules

Two-dimensional van der Waals materials are attractive for photonics and optoelectronics due to distinctive layerdependent optical properties. Optical properties based on light-matter interactions have been revealed by modern imaging and spectroscopy techniques. Hyperspectral imaging microscopy working in line-scan mode (push-broom microspectroscopy) can provide abundant spectral information covering a large area compared to conventional spectroscopy techniques, with a higher acquisition speed than point-scan techniques such as atomic force microscopy and Raman imaging microscopy. This contribution studies in-depth the reconstruction of 3D datacubes and the extraction of optical responses of the sample. Monolayer MoS2, a subclass of semiconducting two-dimensional materials, is fabricated by the mechanical exfoliation method on the SiO2/Si substrate with an oxide thickness of 285 nm. The isolated monolayer MoS2 is observed and identified by a conventional optical microscope. The custom-built push-broom microspectroscope is utilized to scan the region of interest, with the whole spectrum of every line recorded at each frame. The spectral information of every point is collected and 3D spectral data sets are reconstructed for feature extraction and property analysis. To realize the thickness mapping of flakes, linear unmixing is employed to calculate the abundance of isolated monolayer MoS2 on the SiO2/Si substrate, improving flake identification performances. The characteristic spectrum of monolayer MoS2 is acquired by averaging the spectrum from the monolayer MoS2 flake. Furthermore, the optical dielectric response is further analyzed by Kramers-Kronig constrained analysis and Fresnel-law-based analysis. The optical dielectric function is calculated and compared based on the refractive index and medium thickness. This detailed analysis of optical dielectric responses highlights the feasibility of push-broom microspectroscopy for two-dimensional materials characterization.

Graphene and MoS2 are considered to be among the most promising candidates for next generation of electronic and photonic devices. Accurate optical properties provide fundamental information necessary for the rational design of electronic and photonic devices and intended applications. In this abstract, we report the findings from broad band optical measurements of the dielectric function by spectroscopic ellipsometry (SE) on graphene and MoS2 grown by chemical vapor deposition (CVD). The extended spectral range (1-9 eV) afforded by our measurement instrumentation permits us to observe previously unreported new optical critical point transitions and excitons . Monolayer graphene grown on a copper foil was transferred onto a fused silica substrate by using a copper etch, cleaning, and float method previously reported on by our group [1-3]; two and three graphene layers were formed by the sequential transfer of each monolayer. The use of thick fused silica as the substrate is to optimize the measurement sensitivity to a monolayer film thickness. The thickness of 3.35, 6.7 and 10.05 were used for the SE data fitting of monolayer, two-layer and three-layer graphene films, respectively. An exciton transition at 4.8 eV and two additional higher energy absorption peaks at 6.3 eV and 8.5 eV were observed from the absorption spectra of graphene. The peak positions correspond to the resonant excitons that arise from dipole transitions of the single-particle continuum [4]. In the IR region, both refractive index (n) and k increase with wavelength, which is consistent with the previous reported results [5]. It is notable that n increases whereas k decreases as the number of graphene layers increased. This is attributed to the relatively weak van de Waals interaction between transferred graphene monolayers. Such weak interaction is consistent with the relatively small red shift of the 4.8 eV exciton peak we observe in two-layer (0.04 eV shift) and three-layers (0.07 eV shift) graphene relative to that of monolayer graphene. The theory predicts that the transmittance of monolayer graphene depends solely on the universal fine structure constant a = e2/ħc, which is related to the graphene’s opacity. We find the opacity linearly increased for each of added layer. Specifically, at 550 nm wavelength the transmittance of one, two, and three layers of graphene decreases from 96.9%, 93.6%, to 90.3%, respectively. Independent UV-Visible transmittance measurements performed are found to be consistent with the calculated transmittance as obtained from the complex refractive index measured by SE. Monolayer MoS2 grown on SiO2/Si substrate was transferred onto a backside roughed fused silica substrate [6]. Raman spectroscopy was performed on the transferred sample to characterize the monolayer characteristics, and the uniformity and general quality of the film. Optical absorption shows a band gap of 1.81 eV. Photoluminescence (PL) and optical absorption measurements reveal two distinct excitonic transitions at 1.88 eV and 2.02 eV due to direct d-d transitions separated by spin-orbit splitting. The transitions display a blue shift compared with bulk MoS2 as a result of the decreased screening. A series of exciton and optical transition peaks in UV energy range are detected at 3.0, 4.2, 4.9, 5.8, 6.8, and 8.4 eV, of which the 3.0 eV peak has the strongest absorption. In conclusion, the optical functions of CVD-grown graphene and monolayer MoS2 are measured by spectrally extended spectroscopic ellipsometry. The UV energy exciton peaks and a set of optical critical points are observed and related to theoretical predictions. The optical functions and the observed absorption peaks we report here provide a better understanding of graphene and monlayer MoS2 optical properties for the design of optoelectronic devices.

Research on electronic and optical properties of pristine and Ag/Au/Cu-doped graphene/MoS2 heterostructures

For the atomically thin 2D semiconductors with great potentials in the post Moore's era, the doping is a prerequisite for the device engineering and integration. Limited by the doping approaches, elemental doping of 2D semiconductors has not been fully explored. Here, by using a liquid phase edge epitaxy method which can dissolve the dopants in the liquid phase, we have succeeded in doping MoS2 monolayer with more than 40 elements (including transition metals, lanthanides, and main group elements). In this doping library, rarely observed p-type transport behavior has been obtained in Ti, Zn, and Au doped MoS2 monolayers. While Cu, Ga, Zr, Nb, In, Sn, Hf, Ta, Pb, Bi and rare earth of Eu, Gd, Tm, Lu doped MoS2 monolayers have showed low on/off ratios (<10). The obtained p-type, n-type and metallic MoS2 monolayers enables future all-MoS2 based device integration. Additionally, rare earth of Ce, Nd, Dy and Ho doped MoS2 monolayers have showed soft magnetization with semiconducting transport behavior. At last, DFT calculations have revealed that the preferred doping configurations varies for different elements from Mo-substitution, S-substitution to S-site adsorption.

The introduction of mechanical strain is a well-established technique for engineering electronic and optical properties in a broad range of semiconductor materials. In atomically thin materials such as graphene and mono- or few-layer transition metal dichalcogenides (TMDCs), the deliberate introduction and engineering of strain becomes an even more powerful approach for controlling material properties due to the high levels of elastic strain that can be accommodated and the feasibility of achieving precise, highly inhomogeneous strain distributions at the nanoscale. Full realization of and control over these possibilities will require the ability to characterize and control strain, and accompanying changes in electronic, optical, electromechanical, or other properties, in such materials at the nanoscale. To this end, we have used a variety of proximal probe microscopy and spectroscopy techniques to characterize strain and associated material properties in TMDC materials with nanoscale spatial resolution. We first describe the use of tip-enhanced Raman spectroscopy (TERS) and tip-enhanced photoluminescence (TEPL) to characterize strain and optical properties of atomically thin MoS2 to which local strain has been applied via transfer onto a nanopatterned substrate. In these studies, plasmonic modes at the apex of a metal-coated scanning probe tip are excited by laser illumination at or near the plasmon resonance wavelength, enabling Raman scattering and photoluminescence signals to be detected from nanoscale volumes with precise positional control. In tip-induced resonant Raman spectroscopy of monolayer and bilayer MoS2, we observe large enhancements in Raman signal levels measured for MoS2 associated with excitation of plasmonic gap modes between an Au-coated probe tip and Au substrate surface onto which MoS2 has been transferred. Transitions in exciton photoluminescence intensity between monolayer and bilayer regions of MoS2 are observed and discussed. Significant differences in nanoscale Raman spectra between monolayer and bilayer MoS2 are also observed. The origins of specific resonant Raman peaks, their dependence on MoS2 layer thickness, and spatial resolution associated with the transition in Raman spectra between monolayer and bilayer regions are described. In TERS and TEPL studies of bilayer MoS2 with inhomogeneous strain created by transfer to a nanopatterned Au-coated substrate, we observe clear shifts in Raman peak positions and intensities which we correlate with MoS2 phonon deformation potentials and strain-induced changes in electronic structure and phonon dispersion. We then present an example of previously unobserved physical behavior that can arise in connection with nanoscale variations in strain in atomically thin TMDC materials, specifically in the area of electromechanical coupling – the interplay between strain and dielectric polarization in materials. Piezoelectricity, in which there is a linear relationship between dielectric polarization (or an applied electric field) and strain in materials lacking inversion symmetry, is the most well-known of such phenomena. Flexoelectricity, in which strain gradients induce a dielectric polarization field (or its converse, in which gradients in an applied electric field induce strain), is present in all materials but has been much less thoroughly studied, in large part due to the difficulty of achieving sufficiently large strain gradients in conventional crystalline materials. Atomically thin TMDCs offer unique opportunities to explore these and related phenomena. For example, in-plane piezoelectricity was theoretically predicted and subsequently observed experimentally to occur in monolayer and odd-layer MoS2, arising due to the reduction in crystal symmetry in the atomically thin limit. Here, we discuss piezoresponse force microscopy measurements in which out-of-plane electromechanical response, which we have tentatively attributed to flexoelectricity, is observed in monolayer MoS2 and other TMDCs. The estimated effective flexoelectric coefficients are found to be very consistent with those predicted by a simple physical model, and lead to electromechanical coupling response comparable in size to piezoelectric effects in the same materials.

Multifunctional polymer composite gels have attracted attention because of their high thermal stability, conductivity, mechanical properties, and fast optical response. To enable the simultaneous incorporation of all these different functions into composite gels, the best doping material alternatives are two-dimensional (2D) materials, especially transition metal dichalcogenides (TMD), which have been used in so many applications recently, such as energy storage units, opto-electronic devices and catalysis. They have the capacity to regulate optical, electronic and mechanical properties of basic molecular hydrogels when incorporated into them. In this study, 2D materials (WS2, MoS2 and MoSe2)-doped polyacrylamide (PAAm) gels were prepared via the free radical crosslinking copolymerization technique at room temperature. The gelation process and amount of the gels were investigated depending on the optical properties and band gap energies. Band gap energies of composite gels containing different amounts of TMD were calculated and found to be in the range of 2.48–2.84 eV, which is the characteristic band gap energy range of promising semiconductors. Our results revealed that the microgel growth mechanism and gel point of PAAm composite incorporated with 2D materials can be significantly tailored by the amount of 2D materials. Furthermore, tunable band gap energies of these composite gels are crucial for many applications such as biosensors, cartilage repair, drug delivery, tissue regeneration, wound dressing. Therefore, our study will contribute to the understanding of the correlation between the optical and electronic properties of such composite gels and will help to increase the usage areas so as to obtain multifunctional composite gels.

Atomic-Scale Studies of Overlapping Grain Boundaries between Parallel and Quasi-Parallel Grains in Low-Symmetry Monolayer ReS2

In this work, the effects of intrinsic point defects on the electronic and optical properties of both pristine and p-type-doped AlxGa1-xAs nanowires are investigated through first-principles calculations. The formation energy, band structure, charge density difference, density of states, work function, and optical absorption coefficient are analyzed. Results indicate that defects are more prone to formation in the surface layer of nanowires, with a gradual increase in formation energy as defects migrate toward the core region. Band structure and charge density difference analysis reveal that defects such as Gain, Alin, VAs, AsGa, and AsAl introduce donor impurity within the bandgap. In contrast, defects such as Asin, VGa, VAl, GaAs, and AlAs exhibit acceptor impurity characteristics, leading to an elevated work function and hindered electron emission. The former effectively reduces the electron escape barrier, while the latter increases the work function and hinders electron emission. Moreover, p-type doping promotes the formation of antisite defects while suppressing the generation of interstitial defects and vacancy defects. Notably, as the AsGa defect moves away from the p-type doping center, the local electron compensation efficiency diminishes, and the defect-induced donor energy level continuously shifts toward lower energy, resulting in a gradient reduction of bandgap. Furthermore, all defects can enhance the optical absorption capacity of AlxGa1-xAs nanowire photocathodes in the visible light range, especially for p-type doping nanowires.