C. Petridis

Post-fabrication, in situ laser reduction of graphene oxide devices

We report on post-fabrication, in situ, laser induced reduction of graphene oxide (GO) field effect transistors. Our one-step method is efficient, fast, and elevates the conductivity of GO transistor channels by two orders of magnitude. Compared to other reduction techniques, it is facile and simple since it does not require any stringent experimental conditions. Most importantly, we show here that it can be applied for, in situ, post-fabrication reduction of GO devices without compromising any of its components. The physical properties of the laser-reduced graphene oxide were assessed by micro-Raman and X-ray photoelectron spectroscopy analysis and the electrical properties by electric field effect measurements. The application of this technique in other graphene-based optoelectronic devices, especially those fabricated on inexpensive and temperature sensitive flexible substrates such as plastic, is envisaged.

Laser-Assisted Reduction of Graphene Oxide for Flexible, Large-Area Optoelectronics

This paper reviews recent work on the development and use of a low-temperature, laser-based method for the efficient reduction of graphene oxide (GO) films. The method utilizes a laser beam for the in-situ and nonthermal reduction of solution-processed GO layers onto arbitrary substrates. Compared to other reduction techniques, it is single-step, facile, and can be performed at room temperature in ambient atmosphere without affecting the integrity of the either the graphene lattice or the physical properties of the underling substrate. Using this method, conductive layers of reduced GO with a sheet resistance down to ~700 Ω/sq, are obtained. This is much lower than sheet resistance values reported previously for GO layers reduced by chemical means. As a proof of concept, laser-reduced GO layers were successfully utilized as the transparent anode electrodes in flexible bulk-heterojunction OPVs and as the channel material in field-effect transistors. To the best of our knowledge, this is the only example of an in-situ, postfabrication method for the reduction of GO and its implementation in fully functional opto/electronic devices. The nonthermal nature of the method combined with its simplicity and scalability, makes it very attractive for the manufacturing of future generation large-volume graphene-based opto/electronics.

Mode-hop-free tuning over 80 GHz of an extended cavity diode laser without antireflection coating

We report 80 GHz of mode-hop-free tuning from an extended cavity diode laser (ECDL) without the need for antireflection coating on the diode facet. A large continuous tuning range was achieved by using a simple electronic circuit which simultaneously varies the diode laser current and external cavity length in an appropriate ratio. The ECDL employs a commercial, single mode AlGaAs diode laser, operating at 814 nm, in a Littrow external cavity configuration. The applicability of this technique to the most commonly available single-mode diode lasers without the requirement of antireflection coating, makes it an attractive approach for the attainment of mode-hop-free tuning over extended ranges.

Solution processed reduced graphene oxide electrodes for organic photovoltaics

Since the isolation of free standing graphene in 2004, graphene research has experienced a phenomenal growth. Due to its exceptional electronic, optical and mechanical properties, graphene is believed to be the next wonder material for optoelectronics. The enhanced electrical conductivity, combined with its high transparency in the visible and near-infrared regions of the spectrum, enabled graphene to be an ideal low cost indium-tin oxide (ITO) substitute. Solution-processed reduced graphene oxide combines the unique optoelectrical properties of graphene with large area deposition and flexible substrates rendering it compatible with roll-to-roll manufacturing methods. This paper provides an overview of recent research progress in the application and consequent physical-chemical properties of solution-processed reduced graphene oxide-based films as transparent conductive electrodes (TCEs) in organic photovoltaic (OPV) cells. Reduced graphene oxide (rGO) can be effectively utilized as the TCE in flexible OPVs, where the brittle and expensive ITO is incompatible. The prospects and future research trends in graphene-based TCEs are also discussed.

In situ photo-induced chemical doping of solution-processed graphene oxide for electronic applications

We developed a photochemical method for the simultaneous reduction and doping of graphene oxide (GO) layers through ultraviolet laser irradiation in the presence of a dopant precursor gas. It is shown that a few seconds of irradiation is sufficient to dope the GO lattice, while the doping and reduction levels can be readily controlled upon variation of the irradiation time. Using this method, the simultaneous reduction and doping of GO with chlorine or nitrogen atoms is achieved and confirmed by Raman, FTIR and X-ray photoelectron (XPS) spectroscopy measurements. To demonstrate the potential of the approach for practical applications, the photochemical method was successfully employed for the in situ laser induced modification of prefabricated GO field effect transistors. Real time monitoring of the evolution of charge transport as a function of irradiation time reveals significant changes, a result attributed to the chemical modification of the GO lattice. The facile, rapid and room temperature nature of the photo-induced method proposed here provides unique opportunities for the cost-effective synthesis of bulk amounts of chemically modified GO for a wide range of applications spanning from transistors and sensors to transparent electrodes for lighting and photovoltaic cells.

Continuous-wave pump-enhanced singly resonant optical parametric oscillator pumped by an extended-cavity diode laser

A continuous-wave pump-enhanced singly resonant optical parametric oscillator (OPO) directly pumped by a grating-stabilized external-cavity diode laser operating at ≈810 nm is described. The OPO was based on periodically poled LiNbO3 and could be tuned over 1.06–1.19 μm at the signal and 2.58–3.44 μm at the idler. The OPO threshold was typically 25–30 mW over the observed tuning range. Up to 4 mW of one-directional idler output was obtained for 62 mW of external-cavity diode laser pump power. The high stability of the external-cavity diode-laser pump source allowed locked, single-mode OPO operation for periods of greater than 1 h.

Advanced Photonic Processes for Photovoltaic and Energy Storage Systems

Solar-energy harvesting through photovoltaic (PV) conversion is the most promising technology for long-term renewable energy production. At the same time, significant progress has been made in the development of energy-storage (ES) systems, which are essential components within the cycle of energy generation, transmission, and usage. Toward commercial applications, the enhancement of the performance and competitiveness of PV and ES systems requires the adoption of precise, but simple and low-cost manufacturing solutions, compatible with large-scale and high-throughput production lines. Photonic processes enable cost-efficient, noncontact, highly precise, and selective engineering of materials via photothermal, photochemical, or photophysical routes. Laser-based processes, in particular, provide access to a plethora of processing parameters that can be tuned with a remarkably high degree of precision to enable innovative processing routes that cannot be attained by conventional approaches. The focus here is on the application of advanced light-driven approaches for the fabrication, as well as the synthesis, of materials and components relevant to PV and ES systems. Besides presenting recent advances on recent achievements, the existing limitations are outlined and future possibilities and emerging prospects discussed.

High repetition rate pseudospark trigger generator

We report the design and development of a high repetition rate pseudospark trigger generator for operating pseudospark switches with a low impedance of approximately 50 Omega between the trigger pins. The trigger generator can be operated from a single-shot mode up to a repetition rate of 1 kHz. It produces 3 kV, 1 mus pulses into a 100 Omega load to trigger a single pseudospark gap. Also, with the addition of a self-break spark gap and pulse forming network, the trigger generator can be used to trigger a pseudospark gap with low jitter. In this configuration, it produces 300 ns, 3 kV pulses with rise time of 10 ns. The jitter between trigger pulses applied to the pseudospark is less than 1 ns.

A PORTABLE PULSED NEUTRON GENERATOR

The design and construction of a pulsed plasma focus device to be used as a portable neutron source for material analysis such as explosive detection using gamma spectroscopy is presented. The device is capable of operating at a repetitive rate of a few Hz. When deuterium gas is used, up to 105 neutrons per shot are expected to be produced with a temporal pulse width of a few tens of nanoseconds. The pulsed operation of the device and its portable size are its main advantage in comparison with the existing continuous neutron sources. Parts of the device include the electrical charging unit, the capacitor bank, the spark switch (spark gap), the trigger unit and the vacuum–fuel chamber / anode–cathode. Numerical simulations are used for the simulation of the electrical characteristics of the device including the scaling of the capacitor bank energies with total current, the pinch current, and the scaling of neutron yields with energies and currents. The MCNPX code is used to simulate the moderation of the produced neutrons in a simplified geometry and subsequently, the interaction of thermal neutrons with a test target and the corresponding prompt γ-ray generation.

Filamentary Structure of Current Sheath in Miniature Plasma Focus

Current sheath dynamics is an important parameter for good focusing or pinching in a dense plasma focus device. In this paper, we present pinching evidence and details of the filamentary structure of the current sheath in a miniature plasma focus device using a time-resolved laser shadowgraphic technique and time-integrated optical imaging.