Si and Ge allotrope heterostructured nanowires: experimental evaluation of the thermal conductivity reduction.

We have studied the thermal conductivity of Ge and Si allotrope heterostructured nanowires (NWs) synthesized by phase transformation. The NWs are composed of successive hexagonal 2H and cubic diamond 3C crystal phases along the 〈111〉 axis. Using 3ω-scanning thermal microscopy on NWs embedded in a silica matrix, we present the first experimental evidence of thermal conductivity reduction in such allotrope 2H/3C heterostructured NWs. In Ge heterostructured 2H/3C NWs, similarly to homogeneous 3C NWs, we show a thermal conductivity reduction when the NW diameter decreases. In addition, in Si and Ge NWs, we observe a reduced thermal conductivity due to the heterostructuration 2H/3C. We evidence that the temperature of phase transformation, which influences the size and the number of 2H domains, can constitute an efficient parameter to tune the thermal conductivity.

Imaging Thermoelectric Properties at the Nanoscale.

Based on our previous experimental AFM set-up specially designed for thermal conductivity measurements at the nanoscale, we have developed and validated a prototype which offers two major advantages. On the one hand, we can simultaneously detect various voltages, providing, at the same time, both thermal and electrical properties (thermal conductivity, electrical conductivity and Seebeck coefficient). On the other hand, the AFM approach enables sufficient spatial resolution to produce images of nanostructures such as nanowires (NWs). After a software and hardware validation, we show the consistency of the signals measured on a gold layer on a silicon substrate. Finally, we demonstrate that the imaging of Ge NWs can be achieved with the possibility to extract physical properties such as electrical conductivity and Seebeck coefficient, paving the way to a quantitative estimation of the figure of merit of nanostructures.

Correction: Decrease in thermal conductivity in polymeric P3HT nanowires by size-reduction induced by crystal orientation: new approaches towards thermal transport engineering of organic materials.

Correction for 'Decrease in thermal conductivity in polymeric P3HT nanowires by size-reduction induced by crystal orientation: new approaches towards thermal transport engineering of organic materials' by Miguel Muñoz Rojo et al., Nanoscale, 2014, 6, 7858-7865.

Decrease in thermal conductivity in polymeric P3HT nanowires by size-reduction induced by crystal orientation: new approaches towards thermal transport engineering of organic materials.

To date, there is no experimental characterization of thermal conductivity of semiconductor polymeric individual nanowires embedded in a matrix. This work reports on scanning thermal microscopy measurements in a 3ω configuration to determine how the thermal conductivity of individual nanowires made of a model conjugated polymer (P3HT) is modified when decreasing their diameters. We observe a reduction of thermal conductivity, from λNW = 2.29 ± 0.15 W K(-1) m(-1) to λNW = 0.5 ± 0.24 W K(-1) m(-1), when the diameter of nanowires is reduced from 350 nm to 120 nm, which correlates with the polymer crystal orientation measured by WAXS. Through this work, the foundations for future polymer thermal transport engineering are presented.

Nanoscale block copolymer ordering induced by visible interferometric micropatterning: a route towards large scale block copolymer 2D crystals.

We have overcome the cost and time consumption limitations of common lithography techniques used to control the self-assembly of block copolymers into highly ordered 2D arrays through the use of a guiding pattern created from a polymeric sub-layer. The guiding pattern is a sinusoidal surface-relief grating interferometrically inscribed onto an azobenzene containing copolymer sub-layer leading to a defect-free single grain of block copolymer domains.

Thermal exchange radius measurement: application to nanowire thermal imaging.

In scanning thermal microscopy (SThM) techniques, the thermal exchange radius between tip and sample is a crucial parameter. Indeed, it limits the lateral spatial resolution but, in addition, an accurate value of this parameter is necessary for a precise identification of thermal properties. But until now, the thermal exchange radius is usually estimated but not measured. This paper presents an experimental procedure, based on the 3omega-SThM method, to measure its value. We apply this procedure to evaluate the thermal exchange radius of two commercial probes: the well-known Wollaston one and a new probe constituted of a palladium film on a SiO(2) substrate. Finally, presenting silicon nanowire images, we clearly demonstrate that this new probe can reach a spatial resolution better than 100 nm whereas the Wollaston probe hardly reaches a submicronic spatial resolution.

Measurement of thermally induced vibrations of microelectronic devices by use of a heterodyne electronic speckle pattern interferometry imaging technique.

An imaging technique to measure modulated surface displacements on microelectronic devices is presented. A device is supplied by a sinusoidal current that creates a modulated variation of temperature. To measure the induced normal surface displacement, we use an electronic speckle pattern interferometry setup in which we introduce a secondary modulation using an electro-optic modulator. To extract the displacement information, we then analyze the term atthe blinking frequency, which is equal tothe difference between the frequency of the surface displacement and the frequency of the secondary modulation. As the photodetector is a visible CC D camera, weapply heterodyne detection byusing a multichannel lock-in scheme. We have experimented with this new technique on a membrane to measure the amplitude of modulated surface displacement induced by the Joule effect.

Surface displacement imaging by interferometry with a light emitting diode.

We present an imaging technique to measure static surface displacements of electronic components. A device is supplied by a transient current that creates a variation of temperature, thus a surface displacement. To measure the latter, a setup that is based on a Michelson interferometer is used. To avoid the phenomenon of speckle and the drawbacks inherent to it, we use a light emitting diode as the light source for the interferometer. The detector is a visible CCD camera that analyzes the optical signal containing the information of surface displacement of the device. Combining images, we extract the amplitude of the surface displacement. Out-of-plane surface-displacement images of a thermoelectric device are presented.