In this work, we present a comprehensive theoretical and experimental study of quantum confinement in layered platinum diselenide (PtSe 2 ) 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 PtSe 2 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 PtSe 2 energy gap. Large-area PtSe 2 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/SiO 2 substrates. The PtSe 2 films exhibit p-type semiconducting behavior with hole mobility values up to 13 cm 2 /V·s. Metal-oxide-semiconductor field-effect transistors have been fabricated using the grown PtSe 2 films and a gate field-controlled switching performance with an I ON /I OFF ratio of >230 has been measured at room temperature for a 2.5-3 nm PtSe 2 film, while the ratio drops to <2 for 5-6.5 nm-thick PtSe 2 films, consistent with a semiconductingto-semimetallic transition with increasing PtSe 2 film thickness. These experimental observations indicate that the low-temperature growth of semimetallic or semiconducting PtSe 2 could be integrated into the back-end-of-line of a silicon complementary metaloxide-semiconductor process.npj 2D Materials and Applications (2019) 3:33 ; https://doi.

Vibrational spectroscopies, such as Raman and Fouriertransform 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 infraredactive 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-CN 2 ), TIPS-pentacene, and TIPStetraazapentacene 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-CN 2 .

bandgap and exhibit fascinating electrical and optical properties. [1][2][3][4] TMDs have a layered structure in which the transition metal forms a hexagonal layer which is sandwiched between two hexagonal layers of the chalcogen. The TMD monolayers are bonded to each other by weak van der Waals forces, and thus can be considered a 2D material. A number of studies have shown the various applications of TMDs as sensors and in electronic, photonic, and optoelectronic devices. [2,5,6] Another field of critical importance is energy harvesting with thermoelectric power (TEP) which has gained rapid momentum due to the discovery of various nanomaterials. [7][8][9][10][11][12][13][14][15][16][17][18][19] Recently, large thermopower values and efficiencies have been demonstrated in TMDs. [20][21][22][23] Thus, TMDs have great potential as thermoelectric materials; however, most studies have been carried out on mechanically exfoliated TMD mono layers, where it is not possible to control the size and geometry of the device. In order to utilize the high efficiencies and the small device volume of 2D materials for thermoelectric applications, it is an absolute prerequisite to develop a system for homogeneous, large-scale synthesis of TMDs. Thus, the large-scale synthesis of TMDs and device fabrication using such large-scale grown TMD films has become a prominent research area for future semiconductor applications. Recently, TMD films have been synthesized on a large scale by film growth techniques such as chemical vapor deposition (CVD) and thermally assisted conversion (TAC) of the predeposited metal films. [23][24][25][26][27][28][29][30][31] While CVD produces well-defined monolayers, it struggles to produce continuous films. TAC on the other hand produces continuous films of controlled thickness with a polycrystalline morphology. [29,32] Tungsten diselenide (WSe 2 ) is one of the most studied TMDs after molybdenum disulfide (MoS 2 ). Several studies of WSe 2 have discussed its excellent electrical and thermoelectric properties. [4,22,[33][34][35][36] The power factor (S 2 σ) of WSe 2 (≈40 µW K −2 cm −1 ) [22] is comparable with the value of TiS 2 (37 µW K −2 cm −1 ), [37] which is one of the best performing thermoelectric 2D materials. In addition, recently ultralow thermal conductivity in WSe 2 was demonstrated. [38] Thus, a more competitive figure of merit (ZT = S 2 ·σ·T/κ) can be expected for WSe 2 .In this study, WSe 2 films were synthesized on a large scale by TAC on an SiO 2 /Si substrate. The growth of the films takes place 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, WSe 2 films were synthesized on a large scale by thermally assisted conversion (TAC) of W films on SiO 2 /Si substrates at 600 °C. The TAC process yields homogeneous polycrystalline films of controlled thickness over large areas which hav...

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 (MoS 2 ) is critical. Previous approaches for visualizing grains and grain boundaries in MoS 2 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 MoS 2 that are grown on a silicon dioxide (SiO 2 ) substrate. In our approach, the MoS 2 layer on a SiO 2 /Si substrate is exposed to vapor hydrofluoric acid (VHF), resulting in the differential etching of SiO 2 at the MoS 2 grain boundaries and SiO 2 underneath the MoS 2 grains as a result of VHF diffusing through the defects in the MoS 2 layer at the grain boundaries. The location of the grain boundaries can be seen by the resulting SiO 2 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.

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′-[(3,3′″,3″″,4′-tetraoctyl[2,2′:5′,2″:5″,2′″:5′″,2″″-quinquethiophene]-5,5″″-diyl)bis[(Z)-methylidyne(3-ethyl-4-oxo-5,2-thiazolidinediylidene)]]bis-propanedinitrile (DRCN5T) and the acceptor [6,6]-phenyl C71 butyric acid methyl ester (PC71BM) 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 (GIWAXS), electron diffraction, X-ray pole figures, and time-of-flight secondary ion mass spectrometry (ToF-SIMS). A phase separation of DRCN5T and PC71BM 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 a = 19.2 Å, c = 27.1 Å, and β = 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 [010]-direction leads to the extinction of all Bragg reflections with k ≠ 0. The annealed samples are randomly orientated with respect to the normal of the substrate (fiber texture).