Radiative Cooling of Surface-Modified Gold Nanorods upon Pulsed Infrared Photoexcitation

Transient infrared emissions of gold nanorods capped with various materials (AuNR@X, X = CTAB, PSS, mPEG, and SiO2) upon similar to 70-mu s pulsed 1064 nm excitation of their longitudinal surface plasmonic bands were collected with a time-resolved step-scan Fourier-transform spectrometer. Comparing the observed emission contours with the blackbody radiation spectra revealed that parts of the additional emission intensity at low wavenumbers (1300-1000 cm(-1)) were attributed to the vibrational modes of the capping materials, suggesting that the photothermal energy of AuNRs can be thermalized not only via blackbody radiation but also via radiative and nonradiative processes of the capping materials. In addition, the infrared emission of AuNR@SiO2 was more prolonged (similar to 1 ms) than those of the other three (similar to 300 mu s). The photothermal energy can be efficiently randomized to the internal degrees of freedom of the soft molecular capping materials but can be stored by the rigid ones, for example, SiO2, followed by extended radiative cooling.