High-density semiconductor nanorod arrays (NRAs) with one-dimensional (1D) structures have been extensively studied for their application in photonic and electronic devices. [1,2] Especially, 1D periodic NRAs of GaN, ZnO, and ZnS have attracted considerable interest in application to ultraviolet (UV) laser devices due to their direct wide bandgaps of DE g ³ 3.0 eV.[1±4] Among them, ZnO (DE g = 3.37 eV) is thought to be the most suitable material for UV laser devices because of its large exciton binding energy of 60 meV compared to the thermal energy (26 meV) of room temperature.[5]Recently, the room-temperature UV lasing emission from a directionally grown ZnO nanoarray was demonstrated with a threshold power density below 100 kW cm ±2. [1,6] Such NRAs with high-quality UV lasing properties were fabricated only by physical techniques like molecular beam epitaxy (MBE), metal±organic chemical vapor deposition (MOCVD), and gold-catalyzed vapor-phase transport (VPT) techniques; those are, however, expensive and energy consuming processes since they are operated under extreme conditions. [2,7,8] For example, the high-quality ZnO NRA with the best UV-lasing properties was realized by a gold-catalyzed VPT process at 925 C. [8] At this high temperature, however, one can hardly use a silicon (Si) wafer as a substrate, and it is necessary to use a sapphire (Al 2 O 3 ) wafer, even though the Si substrate would be advantageous in terms of easy transformation into electronic devices and low price. It is worth pointing out here that a perfectly and directionally grown ZnO NRA on a Si wafer has been rarely achieved due to the thermal instability of the Si substrate and the large lattice mismatch (~40 %) between the substrate and the ZnO NRA.[9±11]In the present study, a high-quality ZnO NRA was successfully grown on a Si wafer by a wet-chemical process at 95 C for 6 h, where the Si wafer was dip-coated with 4 nm sized ZnO nanoparticles as a buffer and seed layer prior to the crystal growth. To summarize the result in advance, we found that the product ZnO NRA's threshold power density of 70 kW cm ±2 is comparable to the lowest one of 40 kW cm ±2 determined for ZnO NRAs on Al 2 O 3 substrates.[1]ZnO nanoparticles as a starting precursor for the ZnO NRA were prepared according to the previously reported method.[12] As shown in Figure 1a, the particle size of monodispersed ZnO nanoparticles with quasi-spherical shape is determined to be approximately 4 nm. A single particle is observed in detail in the inset of Figure 1a, where the lattice distance between adjacent lattice planes is measured as 5.2 corresponding to the d-spacing between the (0001) planes. The surface image of the present ZnO NRA shows that the nanorod has a well-defined hexagonal plane with a homogeneous diameter of approximately 100 nm due to the uniform growth rate (Fig. 1b). The cross-sectional image in Figure 1c indicates that the nanorods with a uniform length of 1.5 lm are directionally and densely grown over the entire seeded surface of the Si wafer. In add...

Because of its high spatial resolution, scanning thermal microscopy (SThM) has been developed quite actively and applied in such diverse areas as microelectronics, optoelectronics, polymers, and carbon nanotubes for more than a decade since the 1990s. However, despite its long history and diverse areas of application, surprisingly, no quantitative profiling method has been established yet. This is mostly due to the nonlocal nature of measurement by conventional SThM: the signal measured by SThM is induced not only from the local heat flux through the tip-sample thermal contact but also (and mostly) from the heat flux through the air gap between the sample and the SThM probe. In this study, a rigorous but simple and practical theory for quantitative SThM for local measurement is established and verified experimentally using high-performance SThM probes. The development of quantitative SThM will make possible new breakthroughs in diverse fields of nanothermal science and engineering.

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ZnO nanocoral reefs and nanofibers are synthesized on the glass substrate dip coated with ZnO seed with nanoparticles with an average size of 5 nm under a hydrothermal reaction. The ratios of length to diameter for the former and the latter are determined to be 100 and 1000, respectively. In addition, we found that a threshold power density for UV lasing action could be remarkably reduced from 40 kW/cm2 for the nanocoral reefs to 8 kW/cm2 for the nanofibers by increasing the cavity length of ZnO nanowires.

Although scanning thermal microscope has shown the highest spatial resolution in local temperature and thermophysical property measurement, its usefulness has been severely limited due to difficulties in quantitative measurement. We propose a double scan technique that measures temperature only from the heat transfer through the tip-sample contact by the subtraction of the signal due to the heat transfer through the air. A rigorous theoretical model for this technique is derived. The effectiveness of the double scan technique in quantitative temperature measurement is demonstrated experimentally.

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The application of conventional scanning thermal microscopy (SThM) is severely limited by three major problems: (i) distortion of the measured signal due to heat transfer through the air, (ii) the unknown and variable value of the tip-sample thermal contact resistance, and (iii) perturbation of the sample temperature due to the heat flux through the tip-sample thermal contact. Recently, we proposed null-point scanning thermal microscopy (NP SThM) as a way of overcoming these problems in principle by tracking the thermal equilibrium between the end of the SThM tip and the sample surface. However, in order to obtain high spatial resolution, which is the primary motivation for SThM, NP SThM requires an extremely sensitive SThM probe that can trace the vanishingly small heat flux through the tip-sample nano-thermal contact. Herein, we derive a relation between the spatial resolution and the design parameters of a SThM probe, optimize the thermal and electrical design, and develop a batch-fabrication process. We also quantitatively demonstrate significantly improved sensitivity, lower measurement noise, and higher spatial resolution of the fabricated SThM probes. By utilizing the exceptional performance of these fabricated probes, we show that NP SThM can be used to obtain a quantitative temperature profile with nanoscale resolution independent of the changing tip-sample thermal contact resistance and without perturbation of the sample temperature or distortion due to the heat transfer through the air.

Previously, we introduced the double scan technique, which enables quantitative temperature profiling with a scanning thermal microscope (SThM) without distortion arising from heat transfer through the air. However, if the tip-sample thermal conductance is disturbed due to the extremely small size of the sample, such as carbon nanotubes, or an abrupt change in the topography, then quantitative measurement becomes difficult even with the double scan technique. Here, we developed the null-point method by which one can quantitatively measure the temperature of a sample without disturbances arising from the tip-sample thermal conductance, based on the principle of the double scan technique. We first checked the effectiveness and accuracy of the null-point method using 5 μm and 400 nm wide aluminum lines. Then, we quantitatively measured the temperature of electrically heated multiwall carbon nanotubes using the null-point method. Since the null-point method has an extremely high spatial resolution of SThM and is free from disturbance due to the tip-sample thermal contact resistance, and distortion due to heat transfer through the air, the method is expected to be widely applicable for the thermal characterization of many nanomaterials and nanodevices.