Showing papers by "Peter H. Beton published in 2016"

Quantum confinement and photoresponsivity of β-In2Se3 nanosheets grown by physical vapour transport

Abstract: We demonstrate that β-In2Se3 layers with thickness ranging from 2.8 – 100 nm can be grown on SiO2/Si, mica and graphite using a physical vapour transport method. The β-In2Se3 layers are chemically stable at room temperature and exhibit a blue-shift of the photoluminescence emission when the layer thickness is reduced, due to strong quantum confinement of carriers by the physical boundaries of the material. The layers are characterized using Raman spectroscopy and X-ray diffraction from which we confirm lattice constants c = 28.31±0.05 A and a = 3.99±0.02 A. In addition, these layers show high photoresponsivity of up to ~ 2×103 A/W at λ = 633 nm, with rise and decay times of τr = 0.6 ms and τd = 2.5 ms, respectively, confirming the potential of the as-grown layers for high sensitivity, fast photodetectors.

Hexagonal Boron Nitride Tunnel Barriers Grown on Graphite by High Temperature Molecular Beam Epitaxy.

Abstract: We demonstrate direct epitaxial growth of high-quality hexagonal boron nitride (hBN) layers on graphite using high-temperature plasma-assisted molecular beam epitaxy. Atomic force microscopy reveals mono- and few-layer island growth, while conducting atomic force microscopy shows that the grown hBN has a resistance which increases exponentially with the number of layers, and has electrical properties comparable to exfoliated hBN. X-ray photoelectron spectroscopy, Raman microscopy and spectroscopic ellipsometry measurements on hBN confirm the formation of sp2-bonded hBN and a band gap of 5.9 ± 0.1 eV with no chemical intermixing with graphite. We also observe hexagonal moire patterns with a period of 15 nm, consistent with the alignment of the hBN lattice and the graphite substrate.

Strain-Engineered Graphene Grown on Hexagonal Boron Nitride by Molecular Beam Epitaxy

Abstract: Graphene grown by high temperature molecular beam epitaxy on hexagonal boron nitride (hBN) forms continuous domains with dimensions of order 20 μm, and exhibits moire patterns with large periodicities, up to ~30 nm, indicating that the layers are highly strained. Topological defects in the moire patterns are observed and attributed to the relaxation of graphene islands which nucleate at different sites and subsequently coalesce. In addition, cracks are formed leading to strain relaxation, highly anisotropic strain fields, and abrupt boundaries between regions with different moire periods. These cracks can also be formed by modification of the layers with a local probe resulting in the contraction and physical displacement of graphene layers. The Raman spectra of regions with a large moire period reveal split and shifted G and 2D peaks confirming the presence of strain. Our work demonstrates a new approach to the growth of epitaxial graphene and a means of generating and modifying strain in graphene.

High temperature MBE of graphene on sapphire and hexagonal boron nitride flakes on sapphire

Abstract: The discovery of graphene and its remarkable electronic properties has provided scientists with a revolutionary material system for electronics and optoelectronics. Here, the authors investigate molecular beam epitaxy (MBE) as a growth method for graphene layers. The standard dual chamber GENxplor has been specially modified by Veeco to achieve growth temperatures of up to 1850 _C in ultrahigh vacuum conditions and is capable of growth on substrates of up to 3 in. in diameter. To calibrate the growth temperatures, the authors have formed graphene on the Si-face of SiC by heating wafers to temperatures up to 1400 _C and above. To demonstrate the scalability, the authors have formed graphene on SiC substrates with sizes ranging from 10 _ 10mm2 up to 3-in. in diameter. The authors have used a carbon sublimation source to grow graphene on sapphire at substrate temperatures between 1000 and 1650 _C (thermocouple temperatures). The quality of the graphene layers is significantly improved by growing on hexagonal boron nitride (h-BN) substrates. The authors observed a significant difference in the sticking coefficient of carbon on the surfaces of sapphire and h-BN flakes. Our atomic force microscopy measurements reveal the formation of an extended hexagonal moir_e pattern when our MBE layers of graphene on h-BN flakes are grown under optimum conditions. The authors attribute this moir_e pattern to the commensurate growth of crystalline graphene on h-BN.

Naphthalocyanine Thin Films and Field Effect Transistors

Abstract: Naphthalocyanine (Nc) thin films have been grown by sublimation on SiO2. We have used atomic force microscopy and X-ray diffraction to show that the films are disordered for room temperature deposition, but show a highly crystalline needlelike morphology for a substrate temperature of ∼200 °C. Field effect transistors exhibit p-channel operation with a mobility, which has a peak value of 0.052 cm2/(V s), showing a high dependence on substrate temperature. Exposure to atmosphere results in an increase in current and mobility and a reduction in threshold voltage. We compare our results with films formed from analogue molecules such as phthalocyanines and naphthalocyanine functionalized with solubilizing side groups and discuss the potential of Nc for applications in organic electronics and sensors.