Crystalline substrates are known to function as a template for the growth of Van der Waals materials. Van der Waals materials, specifically transition metal dichalcogenides, have gained attention in the past decade due to their interesting chemical and physical properties, as well as the potential they hold for applications, particularly in the field of (opto‐)electronics. The crystalline quality of these materials can be improved by epitaxial growth. Herein, the influence substrate selection has on ordering the growth of MoS 2 is examined. Specifically, the impact sapphire crystal orientation has on the morphology and alignment of MoS 2 grown by chemical vapor deposition is investigated. C‐plane , R‐plane , A‐plane , and M‐plane Al 2 O 3 substrates, annealed at high temperature in air, are used as templates for MoS 2 growth. Comparative analysis, including photoluminescence spectroscopy, Raman spectroscopy, and atomic force microscopy, reveals distinct characteristics and growth modes depending on the growth substrate used. It is found that aligned growth of MoS 2 is influenced by the terrace size, atomic steps, and substrate interaction with the MoS 2 . Aligned growth occurs on both R‐ and M‐plane sapphire. For A‐plane sapphire, the MoS 2 flakes are variable in size and thickness. The weakest substrate interaction is observed for growth on C‐plane sapphire, which consists of randomly oriented flakes.

Grain boundaries in two-dimensional (2D) material layers have an impact on their electrical, optoelectronic, and mechanical properties. Therefore, the availability of simple large-area characterization approaches that can directly visualize grains and grain boundaries in 2D materials such as molybdenum disulfide (MoS2) is critical. Previous approaches for visualizing grains and grain boundaries in MoS2 are typically based on atomic resolution microscopy or optical imaging techniques (i.e., Raman spectroscopy or photoluminescence), which are complex or limited to the characterization of small, micrometer-sized areas. Here, we show a simple approach for an efficient large-area visualization of the grain boundaries in continuous chemical vapor-deposited films and domains of MoS2 that are grown on a silicon dioxide (SiO2) substrate. In our approach, the MoS2 layer on a SiO2/Si substrate is exposed to vapor hydrofluoric acid (VHF), resulting in the differential etching of SiO2 at the MoS2 grain boundaries and SiO2 underneath the MoS2 grains as a result of VHF diffusing through the defects in the MoS2 layer at the grain boundaries. The location of the grain boundaries can be seen by the resulting SiO2 pattern using optical microscopy, scanning electron microscopy, or Raman spectroscopy. This method allows for a simple and rapid evaluation of grain sizes in 2D material films over large areas, thereby potentially facilitating the optimization of synthesis processes and advancing applications of 2D materials in science and technology.

Vibrational spectroscopies, such as Raman and Fourier-transform infrared spectroscopy (FT-IR), are powerful tools for the characterization of organic semiconductor thin films and crystals in addition to X-ray diffraction and scanning atomic force microscopy. They enable the investigation of molecular orientation, polymorphism, doping levels, and intra- as well as intermolecular vibrational modes albeit without much spatial resolution. Two fundamentally different scanning probe techniques offer two-dimensional mapping of infrared-active modes with a spatial resolution below 100 nm: scattering-type scanning near-field optical microscopy (IR s-SNOM) and atomic force microscopy-infrared spectroscopy (AFM-IR). Here, we compare these two techniques with each other and to conventional FT-IR spectroscopy measurements with regard to their applicability to highly ordered molecular semiconductors. For this purpose, we use organic single crystals of rubrene, perfluorobutyldicyanoperylene carboxydiimide (PDIF-CN2), TIPS-pentacene, and TIPS-tetraazapentacene as model systems. We find significant spectral differences depending on the technique and polarization that are related to the anisotropy of the crystals and the fundamentally different working principles of the applied methods. The spatial and spectral resolution of IR s-SNOM and AFM-IR are further tested and compared for a polycrystalline thin film of PDIF-CN2.

It is demonstrated by a detailed structural analysis that the crystallinity and the efficiency of small molecule based organic photovoltaics can be tuned by solvent vapor annealing (SVA). Blends made of the small molecule donor 2,2'-1(3,3'",3",fj'-tetraocty1[2,2':5',2":5",2'":5'",2"quinquethiophene]-5,5"-diy1)bis[(Z)-methylidyne(3-ethyl- oxo-5,2- thiazolid inediylidene)11 bis-propanedinitrile (DRCN5T) and the acceptor [6,61 -phenyl C71 butyric acid methyl ester (PC',BM) were annealed using solvent vapors with either a high solubility for the donor (tetrahydrofuran), the acceptor (carbon disulfide) or both (chloroform). The samples were analyzed by grazing-incidence wide-angle X-ray scattering (CIWAXS), electron diffraction, X-ray pole figures, and time -of-flight secondary ion mass spectrometry (ToF-SIMS). A phase separation of DRCN5T and PC.7,BM is induced by SVA leading to a crystallization of DRCN5T and the formation of a DRCN5T enriched layer. The DRCN5T crystallites possess the two dimensional oblique crystal system with the lattice parameters alpha=19.2 A, c = 27.1 A, and beta = 111.1 for the chloroform case. No major differences in the crystal structure for the other solvent vapors were observed. However, the solvent choice strongly influences the size of the DRCN5T enriched layer. Missing periodicity in the [0101 -direction leads to the extinction of all Bragg reflections with k # O. The annealed samples are randomly orientated with respect to the normal of the substrate (fiber texture).

In this work, we present a comprehensive theoretical and experimental study of quantum confinement in layered platinum diselenide (PtSe2) films as a function of film thickness. Our electrical measurements, in combination with density functional theory calculations, show distinct layer-dependent semimetal-to-semiconductor evolution in PtSe2 films, and highlight the importance of including van der Waals interactions, Green’s function calibration, and screened Coulomb interactions in the determination of the thickness-dependent PtSe2 energy gap. Large-area PtSe2 films of varying thickness (2.5–6.5 nm) were formed at 400 °C by thermally assisted conversion of ultra-thin platinum films on Si/SiO2 substrates. The PtSe2 films exhibit p-type semiconducting behavior with hole mobility values up to 13 cm2/V·s. Metal-oxide-semiconductor field-effect transistors have been fabricated using the grown PtSe2 films and a gate field-controlled switching performance with an ION/IOFF ratio of >230 has been measured at room temperature for a 2.5–3 nm PtSe2 film, while the ratio drops to <2 for 5–6.5 nm-thick PtSe2 films, consistent with a semiconducting-to-semimetallic transition with increasing PtSe2 film thickness. These experimental observations indicate that the low-temperature growth of semimetallic or semiconducting PtSe2 could be integrated into the back-end-of-line of a silicon complementary metal-oxide-semiconductor process.

Of the gases frequently used during graphene chemical vapor deposition (CVD), argon plays no direct chemical role and therefore may be suitable for substitution with the cheaper alternative of nitrogen. The impact of using nitrogen as a carrier gas in methane‐based hot‐wall graphene CVD is investigated using Raman spectroscopy, X‐ray photoelectron spectroscopy, and time‐of‐flight secondary ion mass spectroscopy. No increase in the nitrogen signal is observed within graphene grown within a nitrogen atmosphere within the detection limits of the spectroscopic techniques used.

Platinumdiselenide (PtSe2) is a layered group-ten transition metal dichalcogenide (TMD) with a number of remarkable properties. The bulk semimetal undergoes a transition to semiconductor with decreasing number of atomic layers. The few layered PtSe2 possesses band gaps in the infrared region ideal for multiple applications. Further, PtSe2 can be synthesized at low temperatures and has proven to be relatively air stable, both prerequisites for most applications. In this study, we will outline the synthesis of PtSe2 by thermally assisted conversion (TAC) of prepatterned Pt films [1]. Different methods of Pt deposition are analysed in order to achieve high quality PtSe2 layers. The composition and morphology of the polycrystalline PtSe2 films is investigated by Raman spectroscopy, X-ray photoelectron spectroscopy and scanning probe techniques. Achieving a high film quality is crucial in order to maximize the performance of devices such as chemical sensors [1], IR-Photodetectors [2] or pressure sensors [3].

Owing to their desirable electrical and thermoelectric properties, transition metal dichalcogenides (TMDs) have attracted significant attention. It is important to develop an easy synthetic method and a simple device fabrication process for TMDs. In this study, WSe2 films were synthesized on a large scale by thermally assisted conversion (TAC) of W films on SiO2/Si substrates at 600 °C. The TAC process yields homogeneous polycrystalline films of controlled thickness over large areas which have the advantage that they can be adapted for mass production for applications in electronics and thermoelectrics. In this regard, pre‐patterning of the deposited metal films allows for devices to be easily fabricated without any etch process. UV‐lithography‐defined W structures have been deposited and after conversion to WSe2 their electrical and thermoelectric properties have been studied. Using e‐beam lithography, a field effect transistor (FET) with a WSe2 channel was fabricated. This showed p‐type behavior and reasonable field effect mobility value. The thermoelectric properties of WSe2 thin films were analyzed by additionally integrating micro heating elements to the WSe2 FET. The maximum Seebeck coefficient and power factor (S2·σ) values were calculated to be ≈61 mV·K−1 (Vg = 45 V) and ≈1.3 nW·K−2·cm−1, respectively.