Showing papers by "Aleksandra Krajewska published in 2015"

Residual Metallic Contamination of Transferred Chemical Vapor Deposited Graphene

Abstract: Integration of graphene with Si microelectronics is very appealing by offering a potentially broad range of new functionalities. New materials to be integrated with the Si platform must conform to stringent purity standards. Here, we investigate graphene layers grown on copper foils by chemical vapor deposition and transferred to silicon wafers by wet etching and electrochemical delamination methods with respect to residual submonolayer metallic contaminations. Regardless of the transfer method and associated cleaning scheme, time-of-flight secondary ion mass spectrometry and total reflection X-ray fluorescence measurements indicate that the graphene sheets are contaminated with residual metals (copper, iron) with a concentration exceeding 1013 atoms/cm2. These metal impurities appear to be partially mobile upon thermal treatment, as shown by depth profiling and reduction of the minority charge carrier diffusion length in the silicon substrate. As residual metallic impurities can significantly alter elect...

Sub-90 fs a stretched-pulse mode-locked fiber laser based on a graphene saturable absorber.

Abstract: In this paper a stretched-pulse, mode-locked Er-doped fiber laser based on graphene saturable absorber (SA) is presented. A 60 layer graphene/polymer composite was used as a SA. The all-fiber dispersion managed laser resonator with the repetition frequency of 21.15 MHz allows for Gaussian pulses generation with the full width at half maximum (FWHM) of 48 nm. The generated chirped pulses were compressed outside the cavity to the 88 fs using a piece of standard single mode fiber. The average output power and pulse energy were of 1.5 mW and 71 pJ, respectively.

Multilayer graphene-based saturable absorbers with scalable modulation depth for mode-locked Er- and Tm-doped fiber lasers

Abstract: We demonstrate an experimental study on the influence of the parameters of a graphene-based saturable absorber (SA) on the performance of mode-locked Er- and Tm-doped fiber lasers. We have fabricated a set of saturable absorbers with different number of graphene layers: 9, 12, 24, 37 and 48. Each SA was characterized in terms of nonlinear optical parameters (modulation depth, saturation intensity, saturation fluence) and tested in two state-of-the-art, low-power Er- and Tm-doped fiber lasers. Our results show, that in the Er-laser the broadest output spectrum (11 nm) and shortest pulses (345 fs) are generated using 37 layers of graphene in the SA. In case of a Tm-laser, the best performance (737 fs pulses with 5.82 nm bandwidth) was achieved with 24 layers. Additionally, we show that the modulation depth of a 9-layer SA is insufficient to initiate mode-locking in both lasers. This is the first reported comprehensive study on controlling of the parameters of a SA by scaling the number of graphene layers.

All-polarization maintaining, graphene-based femtosecond Tm-doped all-fiber laser.

Abstract: We report an all-fiber, all-polarization maintaining (PM) ultrafast Tm-doped fiber laser mode-locked by a multilayer graphene-based saturable absorber (SA). The laser emits 603 fs-short pulses centered at 1876 nm wavelength with 6.6 nm of bandwidth and 41 MHz repetition rate. Graphene used as saturable absorber was obtained via chemical vapor deposition (CVD) on copper substrate and immersed in a poly(methylmethacrylate) (PMMA) support, forming a stable, free-standing foil containing 12 graphene layers, suitable for the use in a fiber laser. The generated 603 fs pulses are the shortest reported pulses achieved from a Tm-doped laser mode-locked by graphene saturable absorber so far. Additionally, this is the first demonstration of an all-PM Tm-doped fiber laser incorporating a graphene-based SA. Such cost-effective, compact and stable fiber lasers might be considered as sources usable in nonlinear frequency conversion, mid-infrared spectroscopy and remote sensing.

Application of Turkevich Method for Gold Nanoparticles Synthesis to Fabrication of SiO2@Au and TiO2@Au Core-Shell Nanostructures

Abstract: The Turkevich synthesis method of Au nanoparticles (AuNPs) was adopted for direct fabrication of SiO2@Au and TiO2@Au core-shell nanostructures. In this method, chloroauric acid was reduced with trisodium citrate in the presence of amine-functionalized silica or titania submicroparticles. Core-shells obtained in this way were compared to structures fabricated by mixing of Turkevich AuNPs with amine-functionalized silica or titania submicroparticles. It was found that by modification of reaction conditions of the first method, such as temperature and concentration of reagents, control over gold coverage on silicon dioxide particles has been achieved. Described method under certain conditions allows fabrication of semicontinuous gold films on the surface of silicon dioxide particles. To the best of our knowledge, this is the first report describing use of Turkevich method to direct fabrication of TiO2@Au core-shell nanostructures.

Graphene oxide overprints for flexible and transparent electronics

Abstract: The overprints produced in inkjet technology with graphene oxide dispersion are presented. The graphene oxide ink is developed to be fully compatible with standard industrial printers and polyester substrates. Post-printing chemical reduction procedure is proposed, which leads to the restoration of electrical conductivity without destroying the substrate. The presented results show the outstanding potential of graphene oxide for rapid and cost efficient commercial implementation to production of flexible electronics. Properties of graphene-based electrodes are characterized on the macro- and nano-scale. The observed nano-scale inhomogeneity of overprints' conductivity is found to be essential in the field of future industrial applications.

Graphene and carbon nanocompounds: biofunctionalization and applications in tissue engineering

Abstract: In tissue engineering, the possibility of a comprehensive restoration of the tissue, structure or a portion of the organ is largely determined by the type of material used. A wide range of materials such as graphene and other carbon nanocompounds which have different physical and chemical properties can be expected to react differently upon contact with biomolecules, cells and tissues. This mini-review describes the current knowledge on biocompatibility of graphene and its derivatives with a variety of mammalian cells, such as osteoblasts, neuroendocrine cells, fibroblasts NIH/3T3 line, PMEFs (primary mouse embryonic fibroblasts), stem cells and neurons. The results from different studies give hope for the possibility of graphene to be used in the regeneration of almost all tissues, including neural tissue implants or in the form of neural chips, which may allow in the future treatment of degenerative diseases and injuries of the central nervous system.

260 fs and 1 nJ pulse generation from a compact, mode-locked Tm-doped fiber laser.

Abstract: We report on generation of 260 fs-short pulses with energy of 1.1 nJ from a fully fiberized, monolithic Tm-doped fiber laser system. The design comprises a simple, graphene-based ultrafast oscillator and an integrated all-fiber chirped pulse amplifier (CPA). The system generates 110 mW of average power at 100.25 MHz repetition rate and central wavelength of 1968 nm. This is, to our knowledge, the highest pulse energy generated from a fully fiberized sub-300 fs Tm-doped laser, without the necessity of using grating-based dispersion compensation. Such compact, robust and cost-effective system might serve as a seed source for nonlinear frequency conversion or mid-infrared supercontinuum generation.

Laser ablation- and plasma etching-based patterning of graphene on silicon-on-insulator waveguides.

Abstract: We present a new approach to remove monolayer graphene transferred on top of a silicon-on-insulator (SOI) photonic integrated chip. Femtosecond laser ablation is used for the first time to remove graphene from SOI waveguides, whereas oxygen plasma etching through a metal mask is employed to peel off graphene from the grating couplers attached to the waveguides. We show by means of Raman spectroscopy and atomic force microscopy that the removal of graphene is successful with minimal damage to the underlying SOI waveguides. Finally, we employ both removal techniques to measure the contribution of graphene to the loss of grating-coupled graphene-covered SOI waveguides using the cut-back method.

Method of transferring a graphene film

Abstract: The subject of the invention is a method of transferring a graphene film from a source substrate onto a target substrate, comprising the steps of: marking selected region of the graphene film on the source substrate with a waterproof binder to form a closed frame around the selected region, wherein the source substrate is made of metal, preferably copper, nickel, platinum, rhenium, gold or silver; etching the source substrate until it is completely etched away; depositing the framed graphene film onto the target substrate.

Enhanced Raman scattering and weak localization in graphene deposited on GaN nanowires

Abstract: The influence of GaN nanowires on the optical and electrical properties of graphene deposited on them was studied using Raman spectroscopy and microwave induced electron transport method It was found that interaction with the nanowires induces spectral changes as well as large enhancement of Raman scattering intensity Surprisingly, the smallest enhancement (about 30-fold) was observed for the defect induced D' process and the highest intensity increase (over 50-fold) was found for the 2D transition The observed energy shifts of the G and 2D bands allowed to determine carrier concentration fluctuations induced by GaN nanowires Comparison of Raman scattering spatial intensity maps and the images obtained using scanning electron microscope led to conclusion that vertically aligned GaN nanowires induce a homogenous strain, substantial spatial modulation of carrier concentration in graphene and unexpected homogenous distribution of defects created by interaction with nanowires The analysis of the D and D' peak intensity ratio showed that interaction with nanowires also changes the probability of scattering on different types of defects The Raman studies were correlated with weak localization effect measured using microwave induced contactless electron transport Temperature dependence of weak localization signal showed electron-electron scattering as a main decoherence mechanism with additional, temperature independent scattering reducing coherence length We attributed it to the interaction of electrons in graphene with charges present on the top of nanowires due to spontaneous and piezoelectric polarization of GaN Thus, nanowires act as antennas and generate enhanced near field which can explain the observed large enhancement of Raman scattering intensity

Electron scattering in graphene with adsorbed NaCl nanoparticles

Abstract: In this work, the results of contactless magnetoconductance and Raman spectroscopy measurements performed for a graphene sample after its immersion in NaCl solution were presented. The properties of the immersed sample were compared with those of a non-immersed reference sample. Atomic force microscopy and electron spin resonance experiments confirmed the deposition of NaCl nanoparticles on the graphene surface. A weak localization signal observed using contactless magnetoconductance showed the reduction of the coherence length after NaCl treatment of graphene. Temperature dependence of the coherence length indicated a change from ballistic to diffusive regime in electron transport after NaCl treatment. The main inelastic scattering process was of the electron-electron type but the major reason for the reduction of the coherence length at low temperatures was additional, temperature independent, inelastic scattering. We associate it with spin flip scattering, caused by NaCl nanoparticles present on the graphene surface. Raman spectroscopy showed an increase in the D and D′ bands intensities for graphene after its immersion in NaCl solution. An analysis of the D, D′, and G bands intensities proved that this additional scattering is related to the decoration of vacancies and grain boundaries with NaCl nanoparticles, as well as generation of new on-site defects as a result of the decoration of the graphene surface with NaCl nanoparticles. The observed energy shifts of 2D and G bands indicated that NaCl deposition on the graphene surface did not change carrier concentration, but reduced compressive biaxial strain in the graphene layer.

Multilayer Graphene-based Saturable Absorbers With Scalable Modulation Depth for Mode-Locked Fiber Lasers

Abstract: We present an experimental study on the nonlinear optical properties of multilayer graphene-based saturable absorbers. The influence of the saturable absorber parameters on the performance of mode-locked Erbium and Thulium-doped fiber lasers is investigated.