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 (≥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 Skłodowska-Curie Individual Fellowship (Global) under grant ARTIST (no. 656870) from the European Union's Horizon 2020 research and innovation programme

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) Al$_{2}$O$_{3}$ 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 $\textit{in situ}$ gaseous H$_{2}$O 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)

Graphene-based nanolaminates as ultra-high permeation barriers

Permeation barrier films are critical to a wide range of applications. In particular, for organic electronics and photovoltaics not only ultra-low permeation values are required but also optical transparency. A laminate structure thereby allows synergistic effects between different materials. Here, we report on a combination of chemical vapor deposition (CVD) and atomic layer deposition (ALD) to create in scalable fashion few-layer graphene/aluminium oxide-based nanolaminates. The resulting ~10 nm contiguous, flexible graphene-based films are >90% optically transparent and show water vapor transmission rates below 7 × 10¯³g/m²/day measured over areas of 5 × 5 cm². We deploy these films to provide effective encapsulation for organic light-emitting diodes (OLEDs) with measured half-life times of 880 h in ambient.We acknowledge funding via EPSRC-Innovate UK Grant (EP/M507751/1). B.C.B acknowledges funding from the European Union’s Horizon 2020 research and innovation program under the Marie Skłodowska-Curie Grant Agreements 656214- 2DInterFOX. J.C.M. acknowledges support from the Austrian Science Fund (FWF, P25721-N20)

Copper sulphide (Cu<inf>x</inf>S) as an ammonia gas sensor working at room temperature

A solution growth technique (SGT) has been used to deposit CuxS (x = 1, 1.4, and 2) thin films on glass substrates at room temperature (300 K). These as-deposited thin films are characterized for their structural, optical and electrical properties by X-ray diffraction (XRD), energy dispersive analysis of X-rays (EDAX), scanning electron microscopy (SEM) and atomic force microscopy (AFM), optical absorption and current-voltage (I-V) measurements. XRD shows that the CuxS layer grew with hexagonal and monoclinic phases for x = 1 and 2, respectively. SEM and AFM show the nano-particles (x = 1 and 1.4) and nano-discs (x = 2) formation. The optical band gaps (Eg) of thin films are 1.26 eV (CuS), 1.96 eV (Cu1.4S), and 2.31 eV (Cu2S). In addition, surface wettability is studied by using double-distilled water drops for contact angle measurements. It is observed that the contact angle for Cu1.4S is larger than those for CuS and Cu2S films. It suggests that the x = 1.4 films have high-surface energy. Ammonia gas sensors are fabricated by using these copper sulphide thin films with silver metal contacts. Based on the time-dependent experimental results nanostructured CuxS serve as sensor material for the detection of NH3 molecules at room temperature. © 2008 Elsevier B.V. All rights reserved

Intricate nature of Pd nanocrystal-hydrogen interaction investigated using thermolysed Pd hexadecylthiolate films

Nanocrystalline Pd films are derived by thermolysing Pd hexadecylthiolate at different temperatures (195-250 °C) to obtain films with different resistances, 3-115 Ω. The films essentially consist of Pd nanocrystals (5-40 nm) amidst amorphous carbon as evidenced by electron microscopy and Raman measurements. The response to H2 is seen as a jump in the resistance which varied proportionally with the base resistance, the slope being ∼0.2. Interestingly, the change over from H2 to purging N2 (or Ar) atmosphere is accompanied by a kink-like feature in the electrical response, which is linked to depletion of hydrogen from the nanocrystal surface and its backfilling from the core. The presence of O2 in the purging atmosphere affects adversely the formation of the kink. © 2010 Elsevier B.V

A low cost optical hydrogen sensing device using nanocrystalline Pd grating

A Pd grating of periodicity of 1.5 μm comprising of 1 μm wide nanocrystalline Pd lines has been obtained by a direct micromolding method to serve as Hydrogen sensor element in an optical diffraction set up. The device uses a low power diode laser and a photodetector and works with sensitivity of ∼20%. The hydrogen sensing action is based on monitoring the changes in the diffraction efficiency (DE) which is defined as the ratio of the first and the zeroth order diffracted beam intensities. The diffraction efficiency undergoes large and sudden changes as the nanocrystalline grating becomes disordered due to PdH x formation, as monitored using in-situ microscopy and optical profilometric measurements. This is truly a low cost, portable hydrogen sensor meant for large installations. © 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved

Flexible and semitransparent strain sensors based on micromolded Pd nanoparticle-carbon μ-stripes

Flexible resistive strain sensors have been fabricated by micromolding Pd alkanethiolate on polyimide substrates and subjecting to thermolysis in air. Thus produced stripes were ∼1 μm wide with spacing of ∼0.5 μm and contained Pd nanoparticles in carbon matrix. The nanoparticle size and the nature of carbon are much dependent on the thermolysis temperature as is also the resistance of the microstripes. Generally, lower thermolysis temperatures (<230 °C) produced stripes containing small Pd nanoparticles with significant fraction of carbon from the precursor decomposition. The stripes were poorly conducting yet interestingly, exhibited change of resistance under tensile and compressive strain. Particularly noteworthy are the stripes produced from 195 °C thermolysis, which showed a high gauge factor of ∼390 with strain sensitivity, 0.09%. With molding at 230 °C, the stripes obtained were highly conducting, and amazingly did not change the resistance with strain even after several bending cycles. The latter are ideal as flexible conduits and interconnects. Thus, the article reports a method of producing flexible sensitive strain sensors on one hand and on the other, flexible conduits with unchanging resistance, merely by fine-tuning the precursor decomposition under the molding conditions. © 2011 American Chemical Society

Effect of different substrates on performance of copper sulfide thin film ammonia gas sensor

Copper sulfide (Cu1.4S) thin films have been grown on Silicon (Si), silicon dioxide (SiO2), stainless steel (SS) and fluorine doped tin oxide (FTO) coated glass substrates by solution growth technique (SGT) at room temperature. The effect of substrate on surface morphology is studied by probing surface morphology using scanning electron microscope (SEM) and atomic force microscope (AFM). It is observed that particle size of copper sulfide is larger on silicon and smaller on FTO substrates. Ammonia gas detection testing is carried out by fabricating resistive sensor element and measuring (in-situ) current-voltage (l-V) curves during gas adsorption. In addition, effect of ambient temperature on response of sensor is also studied. Copyright © 2009 American Scientific Publishers All rights reserved

Ultrafast direct ablative patterning of HOPG by single laser pulses to produce graphene ribbons

Ultrafast, single step and direct patterning of highly oriented pyrolytic graphite (HOPG) is achieved through pulsed laser interference ablation using a near field transmitting phase mask. Periodic arrays of lines are patterned on the HOPG surface over large areas by spatially modulating the laser intensity through the mask. Thus patterned surface serve as a source for multi and few layer graphene ribbons for transferring onto desired substrates using polydimethylsiloxane as transferring agent. The transferred regions are contained with few layer graphene (5-6 layers) ribbons as well as thick graphitic ribbons (30-40 nm), with widths ∼1 μm and lengths of several micrometers. Raman, TEM and electrical measurements have confirmed that the transferred ribbons are highly crystalline in nature. Using combinations of shadow and transmitting phase masks, other patterns such as checker boards and diamond-shaped pits are produced. Ultrafast, large-area and direct patterning of HOPG (highly oriented pyrolytic graphite) is achieved through single laser pulse ablation in presence of a phase mask. The modulating laser intensity can ablate the carbon material periodically to produce different types of patterns on HOPG. AFM topography of those patterns reveals that no modification of the regions adjacent to the periodic ablated patterns. Thus patterned surfaces of HOPG are used as source for graphitic and graphene ribbons enabled by PDMS stamp based transfer. The transferred multi and few layer graphene ribbons are of high quality. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Metal-organic molecular device for non-volatile memory storage

Non-volatile memory devices have been of immense research interest for their use in active memory storage in powered off-state of electronic chips. In literature, various molecules and metal compounds have been investigated in this regard. Molecular memory devices are particularly attractive as they offer the ease of storing multiple memory states in a unique way and also represent ubiquitous choice for miniaturized devices. However, molecules are fragile and thus the device breakdown at nominal voltages during repeated cycles hinders their practical applicability. Here, in this report, a synergetic combination of an organic molecule and an inorganic metal, i.e., a metal-organic complex, namely, palladium hexadecylthiolate is investigated for memory device characteristics. Palladium hexadecylthiolate following partial thermolysis is converted to a molecular nanocomposite of Pd(II), Pd(0), and long chain hydrocarbons, which is shown to exhibit non-volatile memory characteristics with exceptional stability and retention. The devices are all solution-processed and the memory action stems from filament formation across the pre-formed cracks in the nanocomposite film. © 2014 AIP Publishing LLC