Fumio S. Ohuchi

van der Waals epitaxial growth and characterization of MoSe2 thin films on SnS2

A variation on molecular beam epitaxy (MBE), called van der Waals epitaxy, is described where a material with primarily two‐dimensional (2D) bonding is grown on a substrate which also has a 2D structure. Lattice matching difficulties, which limit the choice of materials in MBE of 3D systems, are circumvented since the interlayer bonding is from weak van der Waals interactions. The title system shows a lattice mismatch of 10% yet high quality epitaxial films can be grown. The films were characterized in situ with reflection high energy electron diffraction, Auger electron spectroscopy, and low energy electron loss spectroscopy. Additional characterization after exposure to ambient by x‐ray photoelectron spectroscopy, low energy electron diffraction, transmission electron microscopy confirmed the highly ordered nature of the films. Scanning tunneling microscopy provided real space images of the morphology of the epitaxial layer and showed unusual structures attributed to lattice mismatch.

Temperature dependence of the electronic structure of oxides : MgO, MgAl2O4 and Al2O3

We have studied the room temperature optical reflectivity of MgO, MgAl2O4, and α-Al2O3 from 5 to 40eV using a novel spectrophotometer with a laser plasma light source. Structure in the imaginary component of the dielectric response is analysed using critical point line shapes, and the origins of the major transitions in MgO and MgAl2O4 are determined using an ab initio pseudofunction band structure calculation of MgO. The exciton reflectivity has been studied in the three materials at temperatures between 300 and 1500 K, and exciton-phonon coupling appears to increase from MgO to α-Al2O3. The temperature dependence of the higher lying interband transitions in MgO has been determined to 1100 K, and we find that while the temperature dependence of the onset transitions at Γ and X are nearly identical (− 1.22meV/K at Γ), higher lying transitions have very different temperature dependences. Furthermore with increasing temperature the X point valence band separation increases at a rate of 0.38meV/K, while the conduction band separation at X decreases at −0.41meV/K.

Dielectric surface-controlled low-voltage organic transistors via n-alkyl phosphonic acid self-assembled monolayers on high-k metal oxide.

In this paper, we report on n-alkyl phosphonic acid (PA) self-assembled monolayer (SAM)/hafnium oxide (HfO(2)) hybrid dielectrics utilizing the advantages of SAMs for control over the dielectric/semiconductor interface with those of high-k metal oxides for low-voltage organic thin film transistors (OTFTs). By systematically varying the number of carbon atoms of the n-alkyl PA SAM from six to eighteen on HfO(2) with stable and low leakage current density, we observe how the structural nature of the SAM affects the thin-film crystal structure and morphology, and subsequent device performance of low-voltage pentacene based OTFTs. We find that two primary structural factors of the SAM play a critical role in optimizing the device electrical characteristics, namely, the order/disorder of the SAM and its physical thickness. High saturation-field-effect mobilities result at a balance between disordered SAMs to promote large pentacene grains and thick SAMs to aid in physically buffering the charge carriers in pentacene from the adverse effects of the underlying high-k oxide. Employing the appropriate n-alkyl PA SAM/HfO(2) hybrid dielectrics, pentacene-based OTFTs operate under -2.0 V with low hysteresis, on-off current ratios above 1 x 10(6), threshold voltages below -0.6 V, subthreshold slopes as low as 100 mV dec(-1), and field-effect mobilities as high as 1.8 cm(2) V(-1) s(-1).

Periodic lattice distortions as a result of lattice mismatch in epitaxial films of two-dimensional materials

Epilayers of transition metal dichalcogenides (TMDs) with two‐dimensional structures can be grown with molecular beam epitaxy onto other TMDs substrates without regard to lattice matching. Although there is no strong bonding between the epilayer and the substrate, the van der Waals interaction between the two hexagonally closest packed lattices results in a periodic distortion which, due to electronic effects, is prominently imaged with the scanning tunneling microscope.

Controlling Vertical Morphology within the Active Layer of Organic Photovoltaics Using Poly(3-hexylthiophene) Nanowires and Phenyl-C61-butyric Acid Methyl Ester

In this study, we demonstrate how the vertical morphology of bulk heterojunction solar cells, with an active layer consisting of self-assembled poly(3-hexylthiophene) (P3HT) nanowires and phenyl-C(61)-butyric acid methyl ester (PCBM), can be beneficially influenced. Most device fabrication routes using similar materials employ an annealing step to influence active layer morphology, but this process can create an unfavorable phase migration where P3HT is driven toward the top of the active layer. In contrast, we demonstrate devices that exhibit an increase in relative fullerene concentration at the top of the active layer by introducing the donor phase as a solid nanowire in the active layer solution and altering the pre-spin drying time. X-ray photoelectron spectroscopy and conductive and photoconductive atomic force microscopy provide detailed images of how the surface of the active layer can be influenced; this is done by tracking the concentration and alignment of P3HT and PCBM domains. Using this new procedure, devices are made with power conversion efficiencies surpassing 2%. Additionally, we show that nanowires grown in the presence of the fullerene perform differently than those that are grown and mixed separately; exposure to the nanowire during self-assembly may allow the fullerene to coat nanowire surfaces and influence the photocurrent within the device.

ELECTRONIC STRUCTURES AT THE INTERFACES BETWEEN COPPER PHTHALOCYANINE AND LAYERED MATERIALS

Copper phtalocyanine (CuPc) films with the thickness controlled in molecular scales have been grown epitaxially on (0001) surfaces of layered materials, and electronic interaction at the interfaces have been studied by photoelectron spectroscopy. Materials with different electronic properties having different work functions (Evac) were chosen as the substrates; semiconducting MoTe2 (Evac=4.0 eV), semi-metallic highly oriented pyrolytic graphite (Evac=4.5 eV) and metallic TaSe2 (Evac=5.5 eV). Formation of interface dipole layers was found at CuPc/TaSe2 interfaces and molecular orbitals involved were identified.

Cu deposition on Al2O3 and AlN surfaces: Electronic structure and bonding

We report a photoelectron spectroscopy study of the interaction of copper with Al2O3 and AlN model systems processed under ultrahigh vacuum conditions and compared the intrinsic electronic interactions between the two cases. The evolution of the electronic structure and bonding of Cu to AlN has been further studied using ab initio total energy pseudofunction techniques.

WS2 thin films by metal organic chemical vapor deposition

The metal organic chemical vapor deposition (MOCVD) of WS2 by reaction of H2S with W(CO)6 was determined to be thermodynamically favored over a wide range of deposition conditions. Various degrees of crystalline WS2 films were obtained on Si(1 0 0), and the crystalline orientation, structure and morphology of WS2 thin films in relation to the deposition conditions were studied. Microstructure for MOCVD-grown WS2 thin films is generally characterized by the formation of crystallites with basal planes parallel to the interface (c(=)) for the first few tens of nanometers, followed by the formation of crystallites with their basal planes nonparallel to the substrate (c(‖)). Formation of the c(=)–WS2, and transition of the microstructure from c(=) to c(‖) were studied in detail by X-ray diffraction, transmission electron microscopy and atomic force microscopy. Effects of the preferred orientation on the overall crystallinity of thin films and the state of stress were also investigated by Raman spectroscopy.

Work Function and Photothreshold of Layered Metal Dichalcogenides

Work functions and photothreshold values of various layered metal dichalcogenides ( ZrSe2, HfSe2, NbSe2, 1T-TaS2, 2H-TaS2, MoS2, MoSe2, α-MoTe2, SnS2, SnSe2) have been measured by a photoemission technique. The measured photothreshold values, except for Sn compounds, are compared with the calculated values based on various band models. The agreement between the observed and calculated values is satisfactory for most of the materials.

Interface reactions between titanium thin films and (1¯1 2) sapphire substrates

We have studied the reactivity of Ti with the R-plane (1¯1 2) of sapphire from room temperature to 1250 °C by X-ray photoemission spectroscopy (XPS), transmission electron microscopy (TEM), X-ray diffraction (XRD), and Rutherford backscattering spectroscopy (RBS). The combination of these techniques allowed the interface reactions to be studied from the monolayer regime up to the bulk regime. XPS showed that at room temperature, monolayer coverages of Ti reduced the sapphire surface to form Ti-O and Ti-Al bonds. TEM, XRD, and RBS showed that annealing of room-temperature deposited Ti resulted in an interfacial region consisting of two layers, a Ti3Al[O] layer adjacent to the sapphire and a Ti0.67 [O0.33] layer at the free surface. The growth of the Ti3Al[O] layer had an activation energy of 103.4±25 kJ deg-mole. The nature of the interfacial reaction between Ti and sapphire was similar for Ti coverages from the monolayer to the bulk regime.