Uroš Ralević

Atomic force microscopy based manipulation of graphene using dynamic plowing lithography

Tapping mode atomic force microscopy (AFM) is employed for dynamic plowing lithography of exfoliated graphene on silicon dioxide substrates. The shape of the graphene sheet is determined by the movement of the vibrating AFM probe. There are two possibilities for lithography depending on the applied force. At moderate forces, the AFM tip only deforms the graphene and generates local strain of the order of 0.1%. For sufficiently large forces the AFM tip can hook graphene and then pull it, thus cutting the graphene along the direction of the tip motion. Electrical characterization by AFM based electric force microscopy, Kelvin probe force microscopy and conductive AFM allows us to distinguish between the truly separated islands and those still connected to the surrounding graphene.

Spectroscopic imaging ellipsometry and Fano resonance modeling of graphene

In this work, we have examined the optical properties of exfoliated graphene on an Si/SiO2 substrate using spectroscopic imaging ellipsometry in the visible range (360–800 nm). Measured spectra were analyzed by an optical model based on the Fresnel coefficient equations. The optical model was supported by correlated Raman and atomic force microscopy measurements. The complex refractive index of graphene was obtained by inversion of the measured ellipsometry data. The Fano line-shape was used to parameterize the optical properties. Measurements were highly reliable due to the numerous advantages of the spectroscopic imaging ellipsometric technique combined with the proper choice of substrate and experimental set-up. Thickness maps of the graphene sample were obtained from spatially resolved imaging ellipsometry spectra with a spot size of 1 μm. The data showed the presence of a water layer on the surface of the sample, and the thickness was mapped showing the distribution of water over graphene in ambient ...

Influence of transfer residue on the optical properties of chemical vapor deposited graphene investigated through spectroscopic ellipsometry

In this study, we have examined the effects of transfer residue and sample annealing on the optical properties of chemical vapor deposited graphene, transferred onto a sapphire substrate. The optical absorption of graphene was obtained from point-by-point inversion of spectroscopic ellipsometry measurements in the visible and ultraviolet ranges (250–800 nm). Measured spectra were analyzed by optical models based on the Fresnel coefficient equations. The optical models were supported by correlated Raman, scanning electron microscopy, and atomic force microscopy measurements. The obtained data were phenomenologically described by a Fano model. Our results show that a residue layer left on graphene can significantly increase its optical absorption in the visible range, compared to an annealed sample.

Spectroscopic ellipsometry and the Fano resonance modeling of graphene optical parameters

We investigate the optical response of graphene via spectroscopic ellipsometry in the ultraviolet and visible ranges. The optical conductance of graphene is described by a Fano model. The parameters of this model are extracted from our spectroscopic ellipsometry measurements, and the complex refractive index of graphene is obtained. Graphenes dispersion relation shows that the density of states function has a logarithmic van Hove singularity corresponding to the M point of the Brillouin zone. The exciton binding energy is calculated as the difference between the resonant and the saddle point energies.

Femtosecond laser induced periodic surface structures on multi-layer graphene

In this work, we present an observation of laser induced periodic surface structures (LIPSS) on graphene. LIPSS on other materials have been observed for nearly 50 years, but until now, not on graphene. Our findings for LIPSS on multi-layer graphene were consistent with previous reports of LIPSS on other materials, thus classifying them as high spatial frequency LIPSS. LIPSS on multi-layer graphene were generated in an air environment by a linearly polarized femtosecond laser with excitation wavelength λ of 840 nm, pulse duration τ of ∼150 fs, and a fluence F of ∼4.3–4.4 mJ/cm2. The observed LIPSS were perpendicular to the laser polarization and had dimensions of width w of ∼30–40 nm and length l of ∼0.5–1.5 μm, and spatial periods Λ of ∼70–100 nm (∼λ/8–λ/12), amongst the smallest of spatial periods reported for LIPSS on other materials. The spatial period and width of the LIPSS were shown to decrease for an increased number of laser shots. The experimental results support the leading theory behind high s...

Temperature dependent growth morphologies of parahexaphenyl on SiO2 supported exfoliated graphene

The growth of small conjugated molecules on graphene is of increasing interest, since the latter bears the potential to serve as a transparent electrode for organic solar cells and light emitting diodes. Here, parahexaphenyl thin films have been grown by hot wall epitaxy on SiO2 supported exfoliated graphene. The arising morphologies—studied by atomic force microscopy—exhibit a strong dependence on deposition temperature. At temperatures from 280 K–333 K, islands consisting of almost upright standing molecules and needles composed from lying molecules coexist on the graphene flake. Between 363 and 423 K solely needles—consisting of lying molecules—are present on the graphene. The needles form well-ordered networks with relative orientation angles of ∼30°, ∼60°, and ∼90° reflecting the symmetry of the graphene substrate.

Damage effects on multi-layer graphene from femtosecond laser interaction

We present a study on the damage effects of femtosecond laser interaction on exfoliated multi-layer graphene using the techniques of optical microscopy, atomic force microscopy, and Raman spectroscopy. Various effects of the interaction were observed. The ablation threshold was found to be ?4 mJ cm?2, and was slightly higher in transmission mode than in reflection mode. This work also demonstrates the feasibility of ultrafast laser patterning of exfoliated multi-layer graphene.

Modulating light with graphene embedded into an optical waveguide

We investigate the influence that an embedded graphene layer has on guided modes of optical waveguides using exact numerical calculations and a convenient perturbation theory. The latter is found to be highly accurate allowing the graphene-induced changes of modal propagation constants to be determined as the product of a numerical factor characterizing the modal properties of the bare waveguide and the optical conductivity of graphene. In this manner, the influence of the waveguide geometry and the electro-optical properties of graphene on the modulation efficiency can be considered and optimized separately. This result is then used to illustrate the basic electro-absorptive and electro-refractive modulator design principles on a planar waveguide toy model with realistic parameters.

Influence of a gold substrate on the optical properties of graphene

Spectroscopic ellipsometry combined with measurements of electrical characteristics and Kelvin probe force microscopy have been employed to investigate the interaction between graphene and a gold substrate, and the consequent changes of graphenes complex refractive index on gold. A strong blue shift, as much as 350 meV, of the prominent absorption peak (exciton-shifted M-point Van Hove singularity in the ultraviolet range) of graphene has been observed, with respect to the peak position for the sample on an insulating substrate where this peak was observed at about 4.55 eV. The results show that an interaction between graphene and a gold substrate can be characterized through the change of graphenes optical properties. In addition, the effects that a water layer trapped between graphene and gold during the transfer process has on the charge transfer between graphene and the gold substrate have been investigated.

Role of waveguide geometry in graphene-based electro-absorptive optical modulators

We investigate the variation of the graphene-waveguide coupling strength G, whose magnitude is the crucial parameter of efficient graphene-based electro-absorptive optical modulators, with the waveguide geometry and its graphene coverage. By sweeping a wide geometrical parameter space, we show that G can be improved up to few times by optimising the waveguide width and height. We find that the high values of G cannot be simply related to the modal confinement factor and assess the impact of a small detachment of graphene from vertical waveguide boundaries. Using perturbation theory, we demonstrate that the modulation depth to the insertion loss ratio of a graphene-based modulator is always independent of the geometry and determined by the residual conductivity of graphene.