Nanothermometer using single crystal silver nanospheres.

Conventional thermometers have been widely employed in scientific researches and industrial applications. Thermometers with nanometer scaled spatial resolution attract more and more attentions recently with the rapid development of nanotechnology and nanoengineering. Many kinds of nanothermometers have been designed by duplicating the conventional thermometers at nanoscale through decreasing the geometrical size of the conventional thermometers. For example, nanoscale thermocouples are fabricated from nano-junctions based on Seebeck effect, liquid-in-tube nanothermometers from nanotubes based on temperature-dependent thermal expansion of liquid, nanosized fluorescence thermometers from nanoparticles based on temperature-dependent photoluminescence, nanoscale infrared thermometers from metal nanoparticles based on blackbody radiation, Coulomb Blockade nanothermometers from nanosized superconductor-insulator-metal tunnel junctions based on the Coulomb blockade of tunneling, and complex structured nanothermometers from MicroElectro-Mechanical-Systems based on temperature-dependent resonator quality factor or Fermi-level shift, etc. In all of these nanothermometers, the physical properties such as the voltage in nanoscale thermocouples, volume of liquid in liquid-in-tube nanothermometers, or photoluminescence spectrum in nanosized fluorescence thermometers are restored to their original state at room temperature after the temperature drops from high temperature. These nanothermometers are usually employed in real-time and in situ temperature detection, but not for recording and readout later, which is not useful for the situation where real-time readout is not possible like in the case of explosion, but the open-ended gallium-filled carbon nanotube thermometers can in principle also be a readout device after the event. Here another kind of nanothermometers, ex situ nanothermometers, which can record the temperature they were exposed to and be read later when the event is over, are demonstrated to measure temperature based on temperature-dependent size distribution and areal density of metal nanospheres. Compared with the reported nanothermometers, the nanothermometers made of nanospheres with a nanometer scaled spatial resolution, described in this paper, can record the highest temperature in the event and be read at a later time after the event is over. Figure 1a shows the deposited silver nanoparticles before being heated. Silver nanoparticles aggregate on the carbon supporting film coated on TEM grid. The shape of the nanoparticles is irregular and the size varies from 10nm to 100 nm. After heating to a certain temperature, smaller silver nanoparticles nucleate and grow on the whole carbon film (Fig. 1b). It is speculated that the surface diffusion causes the nucleation and growth of the new nanoparticles. Statistical analysis shows that all the nucleated nanoparticles are spherical with an average circularity M1⁄4 0.82–0.85. So these nucleated nanoparticles are named as nanospheres. The spherical morphology of the nanospheres should come from the surface melting because of melting point depression. The inset in Figure 1c shows a high-resolution TEM (HRTEM) image of a nanosphere with diameter of 16 nm observed at 500 8C. The surface of the silver nanosphere melts at 500 8Cwhile the melting point of bulk silver is 961 8C, a direct observation of the significant melting point reduction of nanosized silver particles. Themelting liquid layer covers the nanospheres, conceals the lowest-energy growth facets, and forms perfect spheres because of surface tension. When the surface-melted nanospheres cool down quickly from heating temperature, the main spherical shape is kept. High magnification TEM images (Fig. 1c) show that the nucleated nanospheres distribute uniformly on the carbon film and the size distribution of the nanospheres is narrow. HRTEM image of the nanospheres (Fig. 1d) indicates that the nanospheres are single crystalline at room temperature. EDS spectrum (Fig. 1e) and selected area electron diffraction of the nanospheres (Fig. 1f) confirm that the nucleated nanospheres are silver with face-centered cubic structure. Figure 2 shows the room temperature TEM images, size distribution, and the average diameter of the nanospheres after heating at different annealing temperatures and cooling down. Each sample was heated at a certain heating temperature for 5min and then cooled down to room temperature for TEM examination. TEM images (Fig. 2a) show that the nucleated nanospheres are smaller with heating at 300 8C than those at 500 8C. Histogram of the nanospheres (Fig. 2b) indicates that the diameter of most nanospheres is 4 nm after heating at 300 8C and 14 nm at 700 8C. The average diameter is systematically larger with higher heating temperature (Fig. 2c) because of coalescence.