04.05.2016
Semiconductor Today : the first choice for professionals who demand timely, focused, top-quality coverage of the compound semiconductor industry. Meijo and Nagoya universities in Japan have developed a laser lift-off (LLO) technique for removing gallium nitride (GaN) substrates from ultraviolet (UV) light-emitting diodes (LEDs) to improve light extraction efficiency [Daisuke Iida et al, Appl. However, UV LED devices cannot be separated in this way since the UV laser light would also damage the UV active region.
The heterostructures were produced by metal-organic vapor phase epitaxy in a face-down 2-inch x 3-wafer horizontal-flow reactor. This would lead to expectations of reduced LED performance for devices grown on sample A templates. Having developed the LLO method, the researchers produced 380nm UV-A LEDs to test its suitability (Figure 1). Finally, the emission surface of the LED was roughened with an etch process in hot potassium hydroxide solution.
Although the LLO device was expected to have reduced internal quantum efficiency (IQE) due to the higher threading dislocation density of the template, the higher light extraction efficiency results in brighter emission compared with a control device produced on a sample B template. The author Mike Cooke is a freelance technology journalist who has worked in the semiconductor and advanced technology sectors since 1997. Disclaimer: Material published within Semiconductor Today and related media does not necessarily reflect the views of the publisher or staff. Science, Technology and Medicine open access publisher.Publish, read and share novel research. Unlike ship sculptures sold onboard, these models are expertly designed to exact ship's lines and architecture. Typical LLO techniques use UV laser light to separate GaN buffer layers from sapphire substrates. Tests suggested that the SL removing layer that had decomposed into indium droplets absorbs visible light through surface plasmon resonance. Also, the peak wavelength of the LLO LED was slightly shorter at 380nm, compared with 383nm for the control.
At the same time, the current versus voltage performance of the two devices was very similar. The most common precursors for graphene and metal compound are functional GO and metal salts, respectively. Normally, the photogenerated charge carriers quickly recombine with only a small fraction of the electrons and holes participating in the photocatalytic reaction, resulting in low conversion efficiency [110,111]. In the comparison sample (B) without removing layer, a 1μm n-GaN layer was grown on the free-standing substrate.
The thick GaN substrate absorbs shorter wavelengths more strongly, shifting the peak to longer wavelengths. What is graphene?Graphene is a flat monolayer of sp2-bonded carbon atoms tightly packed into a two-dimensional (2D) honeycomb lattice. The presence of epoxy and hydroxyl functional groups on graphene can act as the heterogeneous nucleation sites and anchor semiconductor nanoparticles avoiding the agglomeration of the small particles [99].
When graphene was introduced into TiO2 nanocomposite, the photogenerated electrons on the conduction band (CB) of TiO2 tend to transfer to graphene sheets, suppressing the recombination of photogenerated electron-holes. The absorbed light decomposes the GaN structure and droplets of Ga form, allowing separation of the GaN from sapphire.
The use of free-standing GaN substrates reduces the density of performance-killing threading dislocations, also improving performance.


Therefore, the laser light is selectively absorbed by the removing layer, allowing lift-off. It is a basic building block for graphitic materials of all other dimensionalities (see Fig.1 from ref. The device layers were thus not damaged by high-energy visible laser light, as often happens in other LLO processes. It has high thermal conductivity (~5,000 W m?1K?1) [2], excellent mobility of charge carriers (200,000 cm2 V?1 s?1) [3], a large specific surface area (calculated value, 2,630 m2 g?1) [4] and good mechanical stability [5].
Additionally, the surface of graphene is easily functionalized in comparison to carbon nanotubes. Thus, graphene has attracted immense attention [1,6-8] and it shows great applications in various areas such as nanoelectronics, sensors, catalysts and energy conversion since its discovery in 2004 [9-14].
With the concentration of graphene oxide high enough and stirring off, long-range ordered assemblies of TiO2-GO sheets were obtained because of self-assembly. To date, various methods have been developed for the preparation of graphene via chemical or physical routes. Novoselov in 2004 firstly reported the micromechanical exfoliation method to prepare single-layer graphene sheets by repeated peeling [1].
Though the obtained graphene has high quality, micromechanical exfoliation has yielded small samples of graphene that are useful for fundamental study. Then methods such as epitaxial growth and chemical vapor deposition have been developed [15-20]. Applications of Graphene-based Semiconductor Nanocomposites for Photocatalytic Hydrogen EvolutionHydrogen is regarded as an ultimate clean fuel in the future because of its environmental friendliness, renewability, high-energy capability, and a renewable and green energy carrier [103-105].
In epitaxial growth, graphene is produced by decomposition of the surface of silicon carbide (SiC) substrates via sublimation of silicon atoms and graphitization of remaining C atoms by annealing at high temperature (1000-1600°C). Using solar energy to produce hydrogen from water splitting over semiconductor is believed to be a good choice to solve energy shortage and environmental crisis [106,107]. Epitaxial graphene on SiC(0001) has been demonstrated to exhibit high mobilities, especially multilayered films. Various semiconductor photocatalysts have been reported to have the performance of photocatalytic hydrogen evolution from water. Summary and PerspectivesIn summary, graphene can be coupled with various semiconductors to form graphene-semiconductor nanocomposites due to its unique large surface area, high conductivity and carriers mobility, easy functionalization and low cost. Recently, single layered SiC converted graphene over a large area has been reported and shown to exhibit outstanding electrical properties [21].
However, the practical application of this strategy is limited due to the fast recombination of photoinduced electron-holes and low utilization efficiency of visible light.
The unique properties of graphene have opened up new pathways to fabricate high-performance photocatalysts.
Because of the superior electrical property of graphene, there is a great interest in combining semiconductor photocatalysts with graphene to improve their photocatalytic H2 production activity [8,54].
These composites have shown potential applications in energy conversion and environmental treatment areas.Although great progress has been achieved, challenges still exist in this area and further developments are required.
However, the graphene obtained from micromechanical exfoliation and chemical vapor deposition has insufficient functional groups, which makes its dispersion and contact with photocatalysts difficult [22].
The first challenge is that the quality-control issues of graphene still need to be addressed. Among the various preparation methods, the reduction of exfoliated graphene oxide (GO) was proven to be an effective and reliable method to produce graphene owing to its low cost, massive scalability, and especially that the surface properties of the obtained graphene can be adjusted via chemical modification [23].


The influences of graphene loading contents and calcination atmosphere on the photocatalytic performance of the sol-gel prepared TiO2-graphene composites have been investigated, respectively. Graphene oxide is believed to be a better starting material than pure graphene to form nanocomposite with semiconductor photocatalysts.
Thus, the development of functionalized graphene-based nanocomposites has aroused tremendous attraction in many potential applications including energy storage [24], catalysis [25], biosensors [26], molecular imaging [27] and drug delivery [28]. However, reduction of graphene oxide into graphene usually can bring defects and impurity simultaneously.
Thus, new synthesis strategies have to be developed to fabricate high-performance graphene-semiconductor composites.
The introduction of graphene into the nanocomposites mainly acts to promote the separation of charge carriers and transport of photogenerated electrons.
What is photocatalytic hydrogen evolution?Photocatalytic water splitting is a chemical reaction for producing hydrogen by using two major renewable energy resources, namely, water and solar energy.
They investigated the effect of TiO2 precursor on the photocatalytic performance of the synthesized nanocomposites under UV light irradiation. The performance of photocatalysts is highly dependent on the semiconductor photocatalysts and their surface structures such as the morphologies and surface states. As the feedstocks for the reaction, water is clean, inexpensive and available in a virtually inexhaustible reserve, whereas solar energy is also infinitely available, non-polluting and appropriate for the endothermic water splitting reaction. Thus, the utilization of solar energy for the generation of hydrogen from water has been considered as an ultimate solution to solve the crisis of energy shortage and environmental degradation [29]. Furthermore, the underlying mechanism of the photocatalytic enhancement by the graphene-based semiconductor nanocomposites is partly unclear. For example, whether graphene can change the band gap of the semiconductor photocatalysts, and whether graphene can truly sensitize semiconductor photocatalysts. Nevertheless, there are still many challenges and opportunities for graphene-based semiconductor nanocomposites and they are still expected to be developed as potential photocatalysts to address various environmental and energy-related issues.6. It can be observed that TiO2 nanoparticles dispersed uniformly on graphene sheets as shown in Figure 7(A). These nanosized composites exhibited higher H2-production rate than that of pure CdS nanoparticles.
Mechanism of the Enhanced Photocatalytic Performance for H2 EvolutionIt is well-known that graphene has large surface area, excellent conductivity and high carriers mobility. The large surface of graphene sheet possesses more active adsorption sites and photocatalytic reaction centers, which can greatly enlarge the reaction space and enhance photocatalytic activity for hydrogen evolution [74,110].Excellent conductivity and high carriers mobility of graphene sheets facilitate that graphene attached to semiconductor surfaces can efficiently accept and transport electrons from the excited semiconductor, suppressing charge recombination and improving interfacial charge transfer processes.
In the EIS measurements, by applying an AC signal to the system, the current flow through the circuit can be modeled to deduce the electrical behavior of different structures within the system.
Information about the films themselves is obtained from the region between 1 mHz and 1 kHz. At frequencies below 100 Hz, the conductivity is the films themselves, and at ultralow frequencies (1 mHz), the conductivity is dominated by the interface between the film and the FTO. So it can be seen that the RGO in the nanocomposites films not only enhances conductivity within the film but also the conduction between the film and the FTO substrate. The same results are obtained from the inset Nyquist plots, where the radius of each arc is correlated with the charge transfer ability of the corresponding film; the larger the radius the lower the film’s ability to transfer charge.



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