99 research outputs found
Measuring the proton selectivity of graphene membranes
By systematically studying the proton selectivity of free-standing graphene
membranes in aqueous solutions we demonstrate that protons are transported by
passing through defects. We study the current-voltage characteristics of
single-layer graphene grown by chemical vapour deposition (CVD) when a
concentration gradient of HCl exists across it. Our measurements can
unambiguously determine that H+ ions are responsible for the selective part of
the ionic current. By comparing the observed reversal potentials with positive
and negative controls we demonstrate that the as-grown graphene is only weakly
selective for protons. We use atomic layer deposition to block most of the
defects in our CVD graphene. Our results show that a reduction in defect size
decreases the ionic current but increases proton selectivity.This is the author accepted manuscript. The final version is available from AIP via http://dx.doi.org/10.1063/1.493633
Transfer-free graphene passivation of sub 100 nm thin Pt and PtâCu electrodes for memristive devices
Memristive switches are among the most promising building blocks for future neuromorphic computing. These devices are based on a complex interplay of redox reactions on the nanoscale. Nanoionic phenomena enable non-linear and low-power resistance transition in ultra-short programming times. However, when not controlled, the same electrochemical reactions can result in device degradation and instability over time. Two-dimensional barriers have been suggested to precisely manipulate the nanoionic processes. But fabrication-friendly integration of these materials in memristive devices is challenging.Here we report on a novel process for graphene passivation of thin platinum and platinum/copper electrodes. We also studied the level of defects of graphene after deposition of selected oxides that are relevant for memristive switching
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Active metamaterial polarization modulators for the Terahertz frequency range
Abstract
Active control of chirality in the terahertz frequency range is of great importance in many scientific areas, which include research into fundamental optical phenomena, investigation of novel materials, spectroscopy, imaging, wireless communications and chemistry. The lack of efficient, integrated and fast-reconfigurable polarization modulators has hindered, so far, the full exploitation of applications in all the aforementioned fields. Metamaterials are artificial resonant elements possessing unique remarkable properties such as high efficiency and miniaturization capability. The interplay of metallic metamaterial arrays with electrostatically tunable monolayer graphene has been demonstrated to be a valid approach for the realization of a novel class of THz devices. In this work, the realization of active chiral graphene/metamaterial modulator is presented. The versatility of this experimental approach allowed the device integration with broadband sources such as terahertz time domain spectrometers as well as with quantum cascade lasers. A continuous rotation of the polarization plane > 30° has been reported with a reconfiguration speed > 5 MHz. These results pave the way to the integration of fast terahertz polarization modulators in all the applications where these devices are in great demand.</jats:p
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Hybrid graphene nematic liquid crystal light scattering device.
A hybrid graphene nematic liquid crystal (LC) light scattering device is presented. This device exploits the inherent poly-crystallinity of chemical vapour deposited (CVD) graphene films to induce directional anchoring and formation of LC multi-domains. This thereby enables efficient light scattering without the need for crossed polarisers or separate alignment layers/additives. The hybrid LC device exhibits switching thresholds at very low electric fields (< 1 V ÎŒm(-1)) and repeatable, hysteresis free characteristics. This exploitation of LC alignment effects on CVD graphene films enables a new generation of highly efficient nematic LC scattering displays as well as many other possible applications.Funding from the EPSRC (Grant No. EP/K016636/1, GRAPHTED) is acknowledged. P.R.K. acknowledges funding from Cambridge Commonwealth Trust (CCT) and the Lindemann Trust Fellowship. A.A.K would like to thank the Higher Education of Pakistan (HEC) and the CCT for financial support. A.C.V. acknowledges funding from the Cambridge Conacyt Scholarship and the Roberto Rocca Fellowship. A.K. would like to thank the Luys Educational Foundation and Hovnanian Foundation for scholarships.This is the author accepted manuscript. The final version is available from the Royal Society of Chemistry via http://dx.doi.org/10.1039/c5nr04094
Atomic layer deposited oxide films as protective interface layers for integrated graphene transfer
The transfer of chemical vapour deposited (CVD) graphene from its parent growth catalyst has become a bottleneck for many of its emerging applications. The sacrificial polymer layers that are typically deposited onto graphene for mechanical support during transfer are challenging to fully remove and hence leave graphene and subsequent device interfaces contaminated. Here, we report on the use of atomic layer deposited (ALD) oxide films as protective interface and support layers during graphene transfer. The method avoids any direct contact of the graphene with polymers and through the use of thicker ALD layers (â„100nm), polymers can be eliminated from the transfer-process altogether. The ALD film can be kept as a functional device layer, facilitating integrated device manufacturing. We demonstrate back-gated field effect devices based on single-layer graphene transferred with a protective Al2O3 film onto SiO2 that show significantly reduced charge trap and residual carrier densities. We critically discuss the advantages and challenges of processing graphene/ALD bilayer structures.We acknowledge funding from EPSRC (Grant No. EP/K016636/1, GRAPHTED) and ERC (Grant No. 279342, InsituNANO). ACV acknowledges the Conacyt Cambridge Scholarship and Roberto Rocca Fellowship. JAA-W acknowledges the support of his Research Fellowships from the Royal Commission for the Exhibition of 1851 and Churchill College, Cambridge. RSW acknowledges a Research Fellowship from St. John's College, Cambridge and a Marie SkĆodowska-Curie Individual Fellowship (Global) under grant ARTIST (no. 656870) from the European Union's Horizon 2020 research and innovation programme
Amplitude stabilization and active control of a terahertz quantum cascade laser with a graphene loaded split-ring-resonator array
We demonstrate the amplitude stabilization of a 2.85âTHz quantum cascade laser with a graphene loaded split-ring-resonator array acting as an external amplitude modulator. The transmittance of the modulator can be actively changed by modifying the graphene conductivity via electrostatic back-gating. The modulator operates at room temperature and is capable of actively modulating the quantum cascade laser power level and thus stabilizing the power output via a proportional-integral-derivative feedback control loop. The stability was enhanced by more than 10 times through actively tuning the modulation. Furthermore, this approach can be used to externally control the laser power with a high level of stability.This work is supported by funding from the Engineering and Physical Sciences Research Council (Grant No. EP/P021859/1, HyperTerahertzâHigh precision terahertz spectroscopy and microscopy)
Crystal Orientation Dependent Oxidation Modes at the Buried Graphene-Cu Interface.
We combine spatially resolved scanning photoelectron spectroscopy with confocal Raman and optical microscopy to reveal how the oxidation of the buried graphene-Cu interface relates to the Cu crystallographic orientation. We analyze over 100 different graphene covered Cu (high and low index) orientations exposed to air for 2 years. Four general oxidation modes are observed that can be mapped as regions onto the polar plot of Cu surface orientations. These modes are (1) complete, (2) irregular, (3) inhibited, and (4) enhanced wrinkle interface oxidation. We present a comprehensive characterization of these modes, consider the underlying mechanisms, compare air and water mediated oxidation, and discuss this in the context of the diverse prior literature in this area. This understanding incorporates effects from across the wide parameter space of 2D material interface engineering, relevant to key challenges in their emerging applications, ranging from scalable transfer to electronic contacts, encapsulation, and corrosion protection
Reactive intercalation and oxidation at the buried graphene-germanium interface
We explore a number of different electrochemical, wet chemical, and gas phase approaches to study intercalation and oxidation at the buried graphene-Ge interface. While the previous literature focused on the passivation of the Ge surface by chemical vapor deposited graphene, we show that particularly via electrochemical intercalation in a 0.25 N solution of anhydrous sodium acetate in glacial acetic acid, this passivation can be overcome to grow GeO2 under graphene. Angle resolved photoemission spectroscopy, Raman spectroscopy, He ion microscopy, and time-of-flight secondary ion mass spectrometry show that the monolayer graphene remains undamaged and its intrinsic strain is released by the interface oxidation. Graphene acts as a protection layer for the as-grown Ge oxide, and we discuss how these insights can be utilized for new processing approaches.We acknowledge financial support from the EPSRC (EP/K016636/1, EP/P51021X/1) and the Future Photonics Hub - Innovation Partnership Fund (EPSRC EP/L00044X/1). P.B.W. acknowledges EPSRC Cambridge NanoDTC EP/G037221/1. R.S.W. acknowledges funding from the European Unionâs Horizon 2020 research and innovation programme through a EU Marie SkĆodowska-Curie Individual Fellowship (Global) under grant ARTIST (no. 656870). R.W. acknowledges EPSRC Doctoral Training Award (EP/M506485/1)
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Encapsulation of graphene transistors and vertical device integration by interface engineering with atomic layer deposited oxide
We demonstrate a simple, scalable approach to achieve encapsulated graphene transistors with negligible gate hysteresis, low doping levels and enhanced mobility compared to as-fabricated devices. We engineer the interface between graphene and atomic layer deposited (ALD) AlO by tailoring the growth parameters to achieve effective device encapsulation whilst enabling the passivation of charge traps in the underlying gate dielectric. We relate the passivation of charge trap states in the vicinity of the graphene to conformal growth of ALD oxide governed by gaseous HO pretreatments. We demonstrate the long term stability of such encapsulation techniques and the resulting insensitivity towards additional lithography steps to enable vertical device integration of graphene for multi-stacked electronics fabrication.This work was supported by the EPSRC (Grant Nos. EP/K016636/1, GRAPHTED and EP/L020963/1) and the ERC (Grant No. 279342, InsituNANO). JAA-W acknowledges a Research Fellowship from Churchill College, Cambridge. JS acknowledges support from NUDT. ZAVV acknowledges funding from ESPRC grant EP/L016087/1. ACV acknowledges the Conacyt Cambridge Scholarship and the Roberto Rocca Fellowship. RW acknowledges EPSRC Doctoral Training Award (EP/M506485/1)
Fast Room-Temperature Detection of Terahertz Quantum Cascade Lasers with Graphene-Loaded Bow-Tie Plasmonic Antenna Arrays
We present a fast room-temperature terahertz detector based on interdigitated bow-tie antennas contacting graphene. Highly efficient photodetection was achieved by using two metals with different work functions as the arms of a bow-tie antenna contacting graphene. Arrays of the bow-ties were fabricated in order to enhance the responsivity and coupling of the incoming light to the detector, realizing an efficient imaging system. The device has been characterized and tested with a terahertz quantum cascade laser emitting in single frequency around 2 THz, yielding a responsivity of âŒ34 ÎŒA/W and a noise-equivalent power of âŒ1.5 Ă 10 W/Hz.R.D., Y.R., and H.E.B. acknowledge financial support from the Engineering and Physical Sciences Research Council (Grant No. EP/J017671/1, Coherent Terahertz Systems). S.H. acknowledges funding from EPSRC (Grant No. EP/K016636/1, GRAPHTED). H.L. and J.A.Z. acknowledge financial support from the EPSRC (Grant No. EP/L019922/1). J.A.A.-W. acknowledges a Research Fellowship from Churchill College, Cambridge. H.J.J. thanks the Royal Commission for the Exhibition of 1851 for her Research Fellowship.This is the final version of the article. It first appeared from American Chemical Society via https://doi.org/10.1021/acsphotonics.6b0040
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