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A new technique for determining the point spread function, which is required for measuring the surface potential using Kelvin probe microscopy (KPM), is presented. The method involves using a silicon carbide substrate coated with single-layer and bilayer graphene as a test structure and obtaining KPM potential profiles in different directions on the surface. This makes it possible to determine the KPM point spread function, which can be used to perform deconvolution and accurately recover the surface potential. [ABSTRACT FROM AUTHOR]

AlzGa1−zAs layers of various compositions were grown using metalorganic chemical vapor deposition on a GaAs substrate with a pattern of alternating SiO2 mask/window stripes, each 100 µm wide. Microphotoluminescence maps and thickness profiles of AlzGa1−zAs layers that demonstrated the distribution of the growth rate and z in the window were experimentally studied. It was shown that the layer growth rate and the AlAs mole fraction increased continuously from the center to the edge of the window. It was experimentally shown that for a fixed growth time of 10 min, as z increased from 0 to 0.3, the layer thickness difference between the center of the window and the edge increased from 700 Å to 1100 Å, and the maximum change in z between the center of the window and the edge reached Δz 0.016, respectively. Within the framework of the vapor -phase diffusion model, simulations of the spatial distribution of the layer thickness and z across the window were carried out. It was shown that the simulation results were in good agreement with the experimental results for the effective diffusion length D/k: Ga—85 µm, Al—50 µm. [ABSTRACT FROM AUTHOR]

Van der Waals (vdW) heterostructures are promising for next-generation two-dimensional electronic and optoelectronic devices. The performance of such devices is completely determined by the properties of the interface. However, due to a lack of contamination-free fabrication techniques, obtaining an ideal interface is still a challenge. Meanwhile, the efficiency of photodetectors and solar cells is highly dependent on the charge separation on the interface. Thus, the questions on the effect of interfacial conditions on a contact type, charge redistribution, and photoluminescence still exist. In this work, the effect of interfacial conditions on the optical and electronic properties of MoSe2/Au heterostructure is studied. The tip of an atomic force microscope is used to clean the interface and change interfacial conditions. Kelvin probe microscopy revealed that the work function of the MoSe2 monolayer increases by 40 meV, the bilayer by 28 meV, and the trilayer by 12 meV due to charge redistribution after the cleaning. Micro-photoluminescence (μ-PL) investigation shows that the cleaning leads to the fall of photoluminescence intensity of about 75% for a monolayer and 60% for a bilayer. Raman spectroscopy indicates that the cleaning procedure did not damage the MoSe2 flake. It is shown that the presence of interfacial contamination in vdW heterostructures severely affects its electronic and optical properties. The results of the work are of great importance to vdW device fabrication. [ABSTRACT FROM AUTHOR]

The fine structure of the exciton spectrum, containing optically allowed (bright) and forbidden (dark) exciton states, determines the radiation efficiency in nanostructures. Time‐resolved microphotoluminescence in MoS2 monolayers (MLs) and bilayers (BLs), both unstrained and compressively strained, in a wide temperature range (10–300 K), is studied to distinguish between exciton states optically allowed and forbidden, both in spin and in momentum, as well as to estimate their characteristic decay times and contributions to the total radiation intensity. The decay times are found to either increase or decrease with increasing temperature, indicating the lowest bright or lowest dark state, respectively. The results unambiguously show that, in an unstrained ML, the spin‐allowed state is the lowest for a series of A excitons (1.9 eV), with the dark state being <2 meV higher, and that the splitting energy can increase several times at compression. In contrast, in the indirect exciton series in BLs (1.5 eV), the spin‐forbidden state is the lowest, being about 3 meV below the bright one. The strong effect of strain on the exciton spectrum can explain the large scatter among the published data and must be taken into account to realize the desired optical properties of 2D MoS2. [ABSTRACT FROM AUTHOR]

The fine structure of the exciton spectrum, containing optically allowed (bright) and forbidden (dark) exciton states, determines the radiation efficiency in nanostructures. Time‐resolved microphotoluminescence in MoS2 monolayers (MLs) and bilayers (BLs), both unstrained and compressively strained, in a wide temperature range (10–300 K), is studied to distinguish between exciton states optically allowed and forbidden, both in spin and in momentum, as well as to estimate their characteristic decay times and contributions to the total radiation intensity. The decay times are found to either increase or decrease with increasing temperature, indicating the lowest bright or lowest dark state, respectively. The results unambiguously show that, in an unstrained ML, the spin‐allowed state is the lowest for a series of A excitons (1.9 eV), with the dark state being <2 meV higher, and that the splitting energy can increase several times at compression. In contrast, in the indirect exciton series in BLs (1.5 eV), the spin‐forbidden state is the lowest, being about 3 meV below the bright one. The strong effect of strain on the exciton spectrum can explain the large scatter among the published data and must be taken into account to realize the desired optical properties of 2D MoS2. [ABSTRACT FROM AUTHOR]

Atomic force microscopy, Kelvin-probe microscopy and Raman spectroscopy have been used to examine graphene films grown by thermal decomposition of the Si face of semi-insulating substrates of 6H-SiC and 4H-SiC polytypes in the atmosphere of argon. It was demonstrated that the quality of graphene grown on substrates of various polytypes at identical technological growth regimes is about the same. A conclusion was made that the differences in crystal structure between 6H-SiC and 4H-SiC does not lead to significant dissimilarities in the mechanism of sublimation of silicon carbide components from the surface of a crystal and in that of graphene crystallization. [ABSTRACT FROM AUTHOR]

Graphene derivatization to either engineer its physical and chemical properties or overcome the problem of the facile synthesis of nanographenes is a subject of significant attention in the nanomaterials research community. In this paper, we propose a facile and scalable method for the synthesis of thiolated graphene via a two-step liquid-phase treatment of graphene oxide (GO). Employing the core-level methods, the introduction of up to 5.1 at.% of thiols is indicated with the simultaneous rise of the C/O ratio to 16.8. The crumpling of the graphene layer upon thiolation without its perforation is pointed out by microscopic and Raman studies. The conductance of thiolated graphene is revealed to be driven by the Mott hopping mechanism with the sheet resistance values of 2.15 kΩ/sq and dependable on the environment. The preliminary results on the chemiresistive effect of these films upon exposure to ethanol vapors in the mix with dry and humid air are shown. Finally, the work function value and valence band structure of thiolated graphene are analyzed. Taken together, the developed method and findings of the morphology and physics of the thiolated graphene guide the further application of this derivative in energy storage, sensing devices, and smart materials. [ABSTRACT FROM AUTHOR]

Selective area epitaxy (SAE) is widely used in photonic integrated circuits, but there is little information on the use of this technique for the growth of heterostructures in ultra-wide windows. Samples of heterostructures with InGaAs quantum wells (QWs) on GaAs (100) substrates with a pattern of alternating stripes (100-μm-wide SiO2 mask/100-μm-wide window) were grown using metalorganic chemical vapour deposition (MOCVD). It was found that due to a local change in the growth rate of InGaAs QW in the window, the photoluminescence (PL) spectra measured from the edge to the center of the window exhibited maximum blueshifts of 14 and 19 meV at temperatures of 80 K and 300 K, respectively. Using atomic force microscopy, we have demonstrated that the surface morphologies of structures grown using standard epitaxy or SAE under identical MOCVD growth conditions correspond to a step flow growth with a step height of ~1.5 ML or a step bunching growth mode, respectively. In the structures grown with the use of SAE, a strong variation in the surface morphology in an ultra-wide window from its center to the edge was revealed, which is explained by a change in the local misorientation of the layer due to a local change in the growth rate over the width of the window. [ABSTRACT FROM AUTHOR]

Transition metal dichalcogenides (TMDs) are promising for new generation nanophotonics due to their unique optical properties. However, in contrast to direct bandgap TMD monolayers, bulk samples have an indirect bandgap that restricts their application as light emitters. On the other hand, the high refractive index of these materials allows for effective light trapping and the creation of high-Q resonators. In this work, a method for the nanofabrication of microcavities from indirect TMD multilayer flakes, which makes it possible to achieve pronounced resonant photoluminescence enhancement due to the cavity modes, is proposed. Whispering gallery mode (WGM) resonators are fabricated from bulk indirect MoSe 2 using resistless scanning probe lithography. A micro-photoluminescence (μ-PL) investigation revealed the WGM spectra of the resonators with an enhancement factor up to 100. The characteristic features of WGMs are clearly seen from the scattering experiments which are in agreement with the results of numerical simulations. It is shown that the PL spectra in the fabricated microcavities are contributed by two mechanisms demonstrating different temperature dependences. The indirect PL, which is quenched with the temperature decrease, and the direct PL which almost does not depend on the temperature. The results of the work show that the suggested approach has great prospects in nanophotonics.

We investigated multilayer plates prepared by exfoliation from a high-quality MoS 2 crystal and revealed that they represent a new object - a van der Waals homostructure consisting of a bulk core and a few detached monolayers on its surface. This architecture comprising elements with different electron band structures leads to specific luminescence, when the broad emission band from the core is cut by the absorption peaks of strong exciton resonances in the surface monolayers. The exfoliated flakes exhibit strong optical anisotropy. We have observed linear to circular polarization conversion that reaches 15% for normally incident light in transmission geometry. This background effect is due to the fluctuations of the c axis relative to the normal, whereas the pronounced resonance contribution is explained by the polarization anisotropy of the excitons localized in the stripes of the dissected surface monolayers.