29 research outputs found
Understanding and optimising the packing density of perylene bisimide layers on CVD-grown graphene
The non-covalent functionalisation of graphene is an attractive strategy to
alter the surface chemistry of graphene without damaging its superior
electrical and mechanical properties. Using the facile method of aqueous-phase
functionalisation on large-scale CVD-grown graphene, we investigated the
formation of different packing densities in self-assembled monolayers (SAMs) of
perylene bisimide derivatives and related this to the amount of substrate
contamination. We were able to directly observe wet-chemically deposited SAMs
in scanning tunnelling microscopy (STM) on transferred CVD graphene and
revealed that the densely packed perylene ad-layers adsorb with the conjugated
{\pi}-system of the core perpendicular to the graphene substrate. This
elucidation of the non-covalent functionalisation of graphene has major
implications on controlling its surface chemistry and opens new pathways for
adaptable functionalisation in ambient conditions and on the large scale.Comment: 27 pages (including SI), 10 figure
Direct Observationof DegenerateTwo-Photon Absorption and Its Saturation in WS2 and MoS2 Monolayer and Few-Layer Films
The optical nonlinearity of WS2, MoS2 monolayer and few-layer films was
investigated using the Z-scan technique with femtosecond pulses from the
visible to the near infrared. The dependence of nonlinear absorption of the WS2
and MoS2 films on layer number and excitation wavelength was studied
systematically. WS2 with 1~3 layers exhibits a giant two-photon absorption
(TPA) coefficient. Saturation of TPA for WS2 with 1~3 layers and MoS2 with
25~27 layers was observed. The giant nonlinearity of WS2 and MoS2 is attributed
to two dimensional confinement, a giant exciton effect and the band edge
resonance of TPA
Heterojunction Hybrid Devices from Vapor Phase Grown MoS
We investigate a vertically-stacked hybrid photodiode consisting of a thin
n-type molybdenum disulfide (MoS) layer transferred onto p-type silicon.
The fabrication is scalable as the MoS is grown by a controlled and
tunable vapor phase sulfurization process. The obtained large-scale p-n
heterojunction diodes exhibit notable photoconductivity which can be tuned by
modifying the thickness of the MoS layer. The diodes have a broad
spectral response due to direct and indirect band transitions of the nanoscale
MoS. Further, we observe a blue-shift of the spectral response into the
visible range. The results are a significant step towards scalable fabrication
of vertical devices from two-dimensional materials and constitute a new
paradigm for materials engineering.Comment: 23 pages with 4 figures. This article has been published in
Scientific Reports. (26 June 2014, doi:10.1038/srep05458
Wide spectral photoresponse of layered platinum diselenide-based photodiodes
Platinum diselenide (PtSe2) is a group-10 transition metal dichalcogenide (TMD) that has unique electronic properties, in particular a semimetal-to-semiconductor transition when going from bulk to monolayer form. We report on vertical hybrid Schottky barrier diodes (SBDs) of two-dimensional (2D) PtSe2 thin films on crystalline n-type silicon. The diodes have been fabricated by transferring large-scale layered PtSe2 films, synthesized by thermally assisted conversion of predeposited Pt films at back-end-of-the-line CMOS compatible temperatures, onto SiO2/Si substrates. The diodes exhibit obvious rectifying behavior with a photoresponse under illumination. Spectral response analysis reveals a maximum responsivity of 490 mA/W at photon energies above the Si bandgap and relatively weak responsivity, in the range of 0.1–1.5 mA/W, at photon energies below the Si bandgap. In particular, the photoresponsivity of PtSe2 in infrared allows PtSe2 to be utilized as an absorber of infrared light with tunable sensitivity. The results of our study indicate that PtSe2 is a promising option for the development of infrared absorbers and detectors for optoelectronics applications with low-temperature processing conditions
Highly sensitive electromechanical piezoresistive pressure sensors based on large-area layered PtSe films
Two-dimensional (2D) layered materials are ideal for micro- and
nanoelectromechanical systems (MEMS/NEMS) due to their ultimate thinness.
Platinum diselenide (PtSe), an exciting and unexplored 2D transition
metal dichalcogenides (TMD) material, is particularly interesting because its
scalable and low temperature growth process is compatible with silicon
technology. Here, we explore the potential of thin PtSe films as
electromechanical piezoresistive sensors. All experiments have been conducted
with semimetallic PtSe films grown by thermally assisted conversion of Pt
at a CMOS-compatible temperature of 400{\deg}C. We report high negative gauge
factors of up to -84.8 obtained experimentally from PtSe strain gauges in
a bending cantilever beam setup. Integrated NEMS piezoresistive pressure
sensors with freestanding PMMA/PtSe membranes confirm the negative gauge
factor and exhibit very high sensitivity, outperforming previously reported
values by orders of magnitude. We employ density functional theory (DFT)
calculations to understand the origin of the measured negative gauge factor.
Our results suggest PtSe as a very promising candidate for future NEMS
applications, including integration into CMOS production lines.Comment: 33 pages, 5 figures, including supporting information with 10 figure