Carlo Maragliano

Thermal property microscopy with frequency domain thermoreflectance.

A thermal property microscopy technique based on frequency domain thermoreflectance (FDTR) is presented. In FDTR, a periodically modulated laser locally heats a sample while a second probe beam monitors the surface reflectivity, which is related to the thermal properties of the sample with an analytical model. Here, we extend FDTR into an imaging technique capable of producing micrometer-scale maps of several thermophysical properties simultaneously. Thermal phase images are recorded at multiple frequencies chosen for maximum sensitivity to thermal properties of interest according to a thermal model of the sample. The phase versus frequency curves are then fit point-by-point to obtain quantitative thermal property images of various combinations of thermal properties in multilayer samples, including the in-plane and cross-plane thermal conductivities, heat capacity, thermal interface conductance, and film thickness. An FDTR microscope based on two continuous-wave lasers is described, and a sensitivity analysis of the technique to different thermal properties is carried out. As a demonstration, we image ~3 nm of patterned titanium under 100 nm of gold on a silicon substrate, and simultaneously create maps of the thermal interface conductance and substrate thermal conductivity. Results confirm the potential of our technique for imaging and quantifying thermal properties of buried layers, indicating its utility for mapping thermal properties in integrated circuits.

Thermal conductance imaging of graphene contacts

Suspended graphene has the highest measured thermal conductivity of any material at room temperature. However, when graphene is supported by a substrate or encased between two materials, basal-plane heat transfer is suppressed by phonon interactions at the interfaces. We have used frequency domain thermoreflectance to create thermal conductance maps of graphene contacts, obtaining simultaneous measurements of the basal-plane thermal conductivity and cross-plane thermal boundary conductance for 1–7 graphitic layers encased between titanium and silicon dioxide. We find that the basal-plane thermal conductivity is similar to that of graphene supported on silicon dioxide. Our results have implications for heat transfer in two-dimensional material systems, and are relevant for applications such as graphene transistors and other nanoelectronic devices.

Effective AFM cantilever tip size: methods for in-situ determination

In atomic force microscopy (AFM) investigations, knowledge of the cantilever tip radius R is essential for the quantitative interpretation of experimental observables. Here we propose two techniques to rapidly quantify in-situ the effective tip radius of AFM probes. The first method is based on the strong dependency of the minimum value of the free amplitude required to observe a sharp transition from attractive to repulsive force regimes on the AFM probe radius. Specifically, the sharper the tip, the smaller the value of free amplitude required to observe such a transition. The key trait of the second method is to treat the tip?sample system as a capacitor. Provided with an analytical model that takes into account the geometry of the tip?sample?s capacitance, one can quantify the effective size of the tip apex fitting the experimental capacitance versus distance curve. Flowchart-like algorithms, easily implementable on any hardware, are provided for both methods, giving a guideline to AFM practitioners. The methods? robustness is assessed over a wide range of probes of different tip radii R (i.e. 4?<?R?<?50?nm) and geometries. Results obtained from both methods are compared with the nominal values given by manufacturers and verified by acquiring scanning electron microscopy images. Our observations show that while both methods are reliable and robust over the range of tip sizes tested, the critical amplitude method is more accurate for relatively sharp tips (4?nm?<?R?<?10?nm).

EFM data mapped into 2D images of tip-sample contact potential difference and capacitance second derivative

We report a simple technique for mapping Electrostatic Force Microscopy (EFM) bias sweep data into 2D images. The method allows simultaneous probing, in the same scanning area, of the contact potential difference and the second derivative of the capacitance between tip and sample, along with the height information. The only required equipment consists of a microscope with lift-mode EFM capable of phase shift detection. We designate this approach as Scanning Probe Potential Electrostatic Force Microscopy (SPP-EFM). An open-source MATLAB Graphical User Interface (GUI) for images acquisition, processing and analysis has been developed. The technique is tested with Indium Tin Oxide (ITO) and with poly(3-hexylthiophene) (P3HT) nanowires for organic transistor applications.

Optofluidic approaches to stationary tracking optical concentrator systems

High Concentration photovoltaics systems (HCPV) allow for improved efficiency but, due to Etandue conservation, have low optical acceptance. Mechanical tracking is normally employed to maintain the necessary alignment of the system axis with the sun. This, however, prevents HCPV from integration in urban and residential environments. We propose here optofluidic based approaches to achieve a stationary tracking optical concentrator by internal modifications of the system optics based on the manipulation of liquid interfaces or multiphase systems. Transparency induced by phase transitions and electrophoretic driven mechanisms will be discussed. Theoretical framework, multiphysics modeling and preliminary experimental results will be presented.

Quantifying electrostatic force contributions in electrically biased nanoscale interactions

A study of the validity of analytical methods for calculating the electrostatic force interaction in alternating current electrostatic force microscopy is presented. Using a simple harmonic oscillator model, two analytical frameworks aimed at relating the electrostatic force between the cantilever tip and the sample with measurable parameters (amplitude and phase of the cantilever) are derived. The validity of the frameworks is examined based on two parameters that define the oscillation amplitude of the cantilever (tip voltage and tip-sample distance). Results are compared with an analytical model of the electrostatic interaction between tip and sample (tip-sample capacitance) and the range of validity of these two frameworks is provided. Our analysis confirms that the commonly used interpretation of the amplitude and the phase as a measure for the electrostatic force and for the derivative of the electrostatic force is only valid for very small oscillation amplitudes and depends on the tip geometry. Fur...

Multi-wall carbon nanostructured paper: characterization and potential applications definition

In the present work, a free-standing paper-like sample made of a three-dimensional web of multiwall carbon nanotubes that exhibit cross-linking and wall sharing is characterized with respect to its electrical, electrochemical, surface and wetting properties. All these parameters are studied simultaneously to assess the potential of the sample for demanding applications with multiple material requirements. It is observed that many characteristics of the one-dimensional counterpart are retained by the three-dimensional web. In addition, the properties of this nanostructured material are compared against zero-, one- two- and three-dimensional carbon-based materials, while potential applications that require high electrical conductivity and energy storage capabilities and related properties, are discussed.

Single element point focus spectral splitting concentrator with CIGS multiple bandgap solar cells

The combination of optical concentration, spatial spectral splitting and the use of multiple cells of suitable bandgap, could provide a path for high PV conversion efficiency without requiring the use of monolithically integrated multi junction solar cells. We propose a dispersive point focus single element concentrator and spectral splitting optics coupled with multiple cells employing Cu(InxGa1-x)Se2 cells for the mid wavelengths region. The optical element is designed, taking advantage of the dispersion characteristics of the employed material, to concentrate and provide spatial spectral splitting. The component can be realized injection molding and be mass produced at low cost.

Efficiency enhancement in two-cell CIGS photovoltaic system with low-cost optical spectral splitter.

Spectrum splitting represents a valid alternative to multi-junction solar cells for broadband light-to-electricity conversion. While this concept has existed for decades, its adoption at the industrial scale is still stifled by high manufacturing costs and inability to scale to large areas. Here we report the experimental validation of a novel design that could allow the widespread adoption of spectrum splitting as a low-cost approach to high efficiency photovoltaic conversion. Our system consists of a prismatic lens that can be manufactured using the same methods employed for conventional CPV optic production, and two inexpensive CuInGaSe(2) (CIGS) solar cells having different composition and, thus, band gaps. We demonstrate a large improvement in cell efficiency under the splitter and show how this can lead to substantial increases in system output at competitive cost using existing technologies.

Switchable Transparency Optical Element for Reactive Solar Tracking

In this article we discuss an emerging concept for non-mechanical solar tracking that can have a significant impact for the design of next generation concentrator photovoltaics systems. Based on the modification of the optical properties of the concentrator elements instead of their mechanical rearrangement, self-tracking concentrators, with recently demonstrated prototypes, could make the mechanical trackers redundant expanding the scope of application of CPV systems. We propose here a new approach to a reactive-tracking system, analyze its underlying physics and discuss initial experimental and simulation results towards the development of a prototype.