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The in-plane thermal conductivity ( k S i G ) of silicon-doped graphene (SiG) was greatly suppressed primarily due to increased phonon scattering associated with the large mass difference of Si from its host C atoms. For SiG as supported on an 8 nm-thick SiO2 substrate, the measured k S i G represents progressive decrease and saturation with the increase of Si dopants concentration, showing more than an order-of-magnitude reduction from that of supported pristine graphene (PG) and nearly two order-of-magnitude reductions when compared with suspended PG at about 2% doping concentration. The enhanced graphene-substrate conformity through thermal annealing in a vacuum additionally lowers k S i G from that of ambient annealing. The substitutional Si dopants tend to suppress the contribution of temperature-sensitive phonons with long mean free paths and weaken the temperature dependence of k S i G . The presence of Si dopants seems to allow for faster attainment of thermal equilibrium between different heat carriers due to the reduced phonon mean free paths. We believe that SiG holds the possibility of exclusively controlling the thermal properties of graphene, since the substitutional dopants do not violently destruct the hexagonal lattice structure of graphene and may possibly have minimal effects on graphene's electrical properties.

Measuring the thermal conductivity kg of supported graphene is inherently complicated due to uncertainties associated with the heat dissipation into the substrate. We innovate the use of an ultra-thin 8-nm SiO2 substrate to alleviate these uncertainties and thus improve the accuracy of optothermal Raman technique to measure kg of supported graphene. As a result, we present an extensive kg database for a wide temperature range from 325 K to 575 K. Furthermore, we have found that the thermal conductivity of supported graphene before annealing is close to that of suspended graphene at 3000 W m−1 K−1, which is attributable to graphene “suspension” lightly on the substrate roughness, and then progressively decreases over repeated thermal annealing. We elaborate on this annealing-induced kg to occur mainly because of the thermally enhanced graphene-substrate conformity and interfacial scattering by probing the Raman spectroscopic characterization of charge carrier density in graphene and the thermal expansion mismatching strain between graphene and substrate. Repeated thermal annealing also expedites the depletion of intercalated impurities to reduce the graphene-substrate separation distance, which acts to further reduces kg, ultimately to its lower bound under vacuum-annealing. Therefore, manipulating the thermo-mechanical affiliation can offer an alternative route to control the in-plane thermal conductivity of supported graphene.

The grain size of CVD (Chemical Vapor Deposition) graphene was controlled by changing the precursor gas flow rates, operation temperature, and chamber pressure. Graphene of average grain sizes of 4.1 &micro, m, 2.2 &micro, m, and 0.5 &micro, m was synthesized in high quality and full coverage. The possibility to tailor the thermoelectric conversion characteristics of graphene has been exhibited by examining the grain size effect on the three elementary thermal and electrical properties of &sigma, S, and k. Electrical conductivity (&sigma, ) and Seebeck coefficients (S) were measured in a vacuum for supported graphene on SiO2/Si FET (Field Effect Transistor) substrates so that the charge carrier density could be changed by applying a gate voltage (VG). Mobility (&micro, ) values of 529, 459, and 314 cm2/V&middot, s for holes and 1042, 745, and 490 cm2/V&middot, s for electrons for the three grain sizes of 4.1 &micro, m, respectively, were obtained from the slopes of the measured &sigma, vs. VG graphs. The power factor (PF), the electrical portion of the thermoelectric figure of merit (ZT), decreased by about one half as the grain size was decreased, while the thermal conductivity (k) decreased by one quarter for the same grain decrease. Finally, the resulting ZT increased more than two times when the grain size was reduced from 4.1 &micro, m to 0.5 &micro, m.

The detrimental effect of nanoscale hole defects on the in-plane thermal conductivity (k) was first examined for supported CVD graphene. A focused ion beam punctured equally spaced 50-nm diameter holes with different hole spacings (200, 400, and 800 nm) in supported graphene on an 8-nm thin SiO2 substrate. For the relatively low 4.91% porosity, the thermal conductivity showed a significant reduction to 212.6 W/mK from 1045 W/mK in supported graphene with no holes and even more dramatically so from 3500 W/mK in suspended pristine graphene. The thermal conductivity showed an order-of-magnitude faster reduction with increasing porosity compared to the Eucken model, which is based on the diffusive thermal transport reduction due to the void holes on the macroscale. This is believed to be attributed to the enhanced phonon scattering by the nanoscale hole edges and also by the reduced phonon passage length-scale that became comparable to the phonon mean-free-paths. Furthermore, a phenomenological fitting model is presented to comprehensively describe the k dependence on porosity, hole spacing, and the spectral dependence of the phonon mean-free-path in nanoscale holey graphene.

Drop condensation and coalescence is visualized using high-speed Surface Plasmon Resonance (SPR) reflectance microscopy. SPR microscopy is a label-free technique that can characterize thin films (less than 1µm) by detecting the changes in the refractive index of the test medium. The sensing surface is a 50 nm thick gold film on a 2.5 nm thick Ti layer is deposited on a borosilicate substrate. P-polarized monochromatic light (632 nm) is incident on the gold film in a total internal reflection mode. Free electrons in the gold film are excited by the incident light when a resonance condition is met. The result is a decrease in the reflected intensity. Resonance depends upon wavelength, incident angle, and refractive index of prism and test medium. To induce condensation, a water bridge is created between the SPR gold film and an ITO coated glass slide. When the ITO coated slide is heated water evaporates from the bridge and condenses on the gold film. The sequence of images on the process of droplet deposition and drop coalescence are captured at 1500 frames per second. Experiments were conducted at an SPR angle of 44o, which is slightly above the minimum intensity angle for air at 43.8o. Therefore, the brightest and darkest regions correspond to the areas on the gold film covered with bulk water and a very thin film of water, respectively. The thickness of the film is proportional to the intensity of reflected light.

Comparative wettability studies of graphene are conducted for two different nanofluids with opposite surface potentials of +53 mV (45-nm alumina nanoparticles) and −45 mV (28-nm silica nanoparticles), respectively. Aged graphene surface, which has adsorbed abundant hydrocarbon contaminants, shows weak hydrophobicity of about 90° wetting angles for both nanofluids for the tested volume concentration range from 0 to 10 %. For pristine graphene surfaces, however, the contact angle of alumina nanofluids continually increases from 50° to 70° for the same volume concentration increase, but the contact angle of silica nanofluids shows first increase of up to about 1 % concentration and then remains nearly unchanged with further increasing concentration. Since the nanoparticle–graphene interaction at the solid–liquid (SL) interface is expected to be the most crucial in determining the nanofluid wetting angles, the corresponding surface energy $$\gamma_{\text{SL}}$$ is examined from elaboration of $$F_{\text{DLVO}}$$ , the Derjaguin–Landau–Verwey–Overbeek force. The magnitudes of both the repulsive $$F_{\text{DLVO}}$$ on the alumina nanoparticles and the attractive $$F_{\text{DLVO}}$$ on the silica nanoparticles show rapid decreases up to 1 % volume concentration and exhibit slower decreases thereafter. The reduced repulsive $$F_{\text{DLVO}}$$ of the alumina nanoparticle drives the increasing aggregation of nanoparticles on the SL interface with increasing concentration, thus increasing the SL interfacial energy $$\gamma_{\text{SL}}$$ . On the contrary, the reduced attractive $$F_{\text{DLVO}}$$ on the silica nanoparticle retards their aggregation on the SL interface with increasing concentration and slows the increase in $$\gamma_{\text{SL}}$$ , eventually settling on the saturated level of $$\gamma_{\text{SL}}$$ from a certain concentration onwards. These distinctive behaviors of $$\gamma_{\text{SL}}$$ are consistent with the measured contact angles that gradually increase with increasing concentration for the positive surface potential (alumina), but initially increase and then settle for the negative surface potential (silica). This phenomenon strongly supports the critical dependence of nanofluid wetting of pristine graphene on $$F_{\text{DLVO}}$$ in the vicinity of the SL interface.

The surface plasmon resonance (SPR) reflectance imaging technique provides a label-free visualization tool to characterize the near-field fluidic transport properties within 100 nm from the solid surface. The key idea is that the SPR reflectance intensity varies with the near-field refractive index (RI) of the test fluid, which in turn depends on the micro- and nano-fluidic scalar properties such as concentrations, temperatures, and phase changes, occurring in the near-field. As essential knowledge to understand and implement the SPR reflectance imaging technique, this paper presents discussions on the basics of surface plasmon polaritons (SPPs), surface plasmon resonance (SPR), setup of the SPR reflectance imaging system, and the SPR reflectance imaging resolution. The second part of the paper elaborates the applications of the SPR imaging sensor technique in characterizing the near-field fingerprints of nanofluidic evaporative self-assembly.

Reliable determination of the complex refractive index (RI) of graphene inherently requires two independent measurement realizations for two independent unknowns of the real (n(G)) and imaginary (k(G)) components, i.e., RI = n(G) + i k(G). Thus, any single set of measurement realization provides only one constraint that is insufficient to uniquely determine the complex RI of graphene. Tandem uses of two independent measurement techniques, namely the surface plasmon resonance (SPR) angle detection and the attenuated total reflection (ATR) intensity measurement, allow for the unique determination of the complex RI of CVD-synthesized graphene. The presently measured graphene RI is determined to be 2.65 + 1.27i for the E-field oscillating parallel to graphene at 634 nm wavelength, with variations for different numbers of L (1, 3 and 5) remaining within +/- 3%. Thus, our demonstration results for the specified wavelength serve as an impetus to suggest the need for two independent measurement techniques in determining both the real and imaginary RI values for graphene. Additional efforts have been made to characterize graphene layers using the density function theory (DFT): this calculation provides RIG = 2.71 +/- 1.41i.

The wetting and evaporative aggregation of alumina nanofluids (Al2O3) are examined for CVD-synthesized graphene-coated (GC) surfaces that are known as strongly hydrophobic (θcontact ≈ 90°). Our findings are compared to those associated with a hydrophilic cover glass (CG) substrate (θcontact ≈ 45°). The nanofluidic self-assemblies on the GC substrate are elaborately characterized in terms of the droplet wetting/crack formation, the particle migration time over the evaporative time (CR), the Derjaguin-Landau-Verwey-Overbeek forces (FDLVO), and the relative thermal conductivity (KR). The GC substrate forms relatively thicker and larger cracks and requires a longer evaporation time. Both the GC and CG substrates share approximately the same time constant CR, which suggests the formation of coffee-ring patterns for both substrates. The GC shows negative FDLVO, which implies a repulsive force between the nanoparticles and the substrate, and the CG shows a positive FDLVO of attraction. Furthermore, a more than 3 order of magnitude larger thermal conductivity of GC compared to that of CG drives significantly different particle/fluid motions near the drop edge areas between the two substrates.

The total thickness of a graphene sample depends upon the number of individually stacked graphene layers. The corresponding surface plasmon resonance (SPR) reflectance alters the SPR angle, depending on the number of graphene layers. Thus, the correlation between the SPR angle shift and the number of graphene layers allows for a nonintrusive, real-time, and reliable counting of graphene layers. A single-layer graphene (SLG) is synthesized by means of chemical vapor deposition, followed by physical transfer to a thin gold film (48 nm) repeatedly, so that multilayer graphene samples with one, three, and five layers can be prepared. Both the measured SPR angles and the entire reflectance curve profiles successfully distinguish the number of graphene layers.