Nitrogen vacancy (NV) centers are fluorescent defects widely employed for thermometry, most commonly via temperature-dependent shifts of their optically detected magnetic resonance. Recently, all-optical approaches based on temperature-dependent features of the NV center fluorescence spectrum have also gained traction. Excited state lifetime thermometry is an all-optical technique that has been implemented using other fluorophores but has not previously been demonstrated for NV centers in individual nanodiamonds (NDs). Here, we report temperature-dependent excited state lifetime measurements of NV centers in individual NDs between 300 K and 500 K. We measure a 32 ± 7.0% and 35 ± 8.3% average decrease in the excited state lifetimes of individual NDs on silicon and glass substrates, respectively, over this temperature range. A linear approximation applicable to nearly all measured NDs yields temperature coefficients of −2000 ± 240 ppm/K and −2600 ± 280 ppm/K for NDs on silicon and glass, respectively. In addition to all-optical operation, single-ND excited state lifetime thermometry offers ∼100 ns temporal resolution and utilizes time-correlated single photon counting measurements ideally suited to low emission intensities, a limiting factor for other NV center thermometry techniques above 700 K. We demonstrate that atomic force microscope nanomanipulation can position individual NDs at critical locations on a sample of interest, enabling single-point temperature measurements that combine ∼100 ns temporal resolution and ∼100 nm spatial resolution. This work also has broader implications for other single-ND excited state lifetime sensing applications, where care is required to avoid conflating changes in temperature and other environmental parameters.

Thermal conductivity of a nickel-coated tri-wall carbon nanotube was studied using molecular dynamics where both the phonon and electron contributions were considered. Simulations predicted a significant effect of the metal coating on the thermal conductivity, i.e. 50% decrease for 1.2 nm of Ni coating. However, the decreasing rate of the thermal conductivity is minuscule for the metal thicker than 1.6 nm. The smaller thermal conductivity of the metal coating, phonon scattering at the interface, and less impacted heat transfer on the inner tubes of the carbon nanotube rationalized the observed trends.

Operando thermometry can help resolve open questions about the importance of thermal contributions to plasmonic photocatalysis, but identifying high‐fidelity thermometers with the requisite chemical inertness, thermal stability, and spatial resolution remains challenging. Here, it is demonstrated that a single near‐infrared laser can simultaneously excite upconverting nanoparticles (UCNPs) that serve as luminescent thermometers and photocatalyze the dimerization of 4‐nitrothiophenol (4‐NTP), which is employed as a model reaction. Due to its large anti‐Stokes shift, the UCNP thermometry signal naturally separates from the 4‐NTP Raman signal, which is used to monitor the chemical reaction, in the spectral domain. The surface temperature rise of plasmonic substrates under varying illumination intensity is systematically correlated with the reaction progress. Temperature rises exceeding 40 K are recorded at the maximum intensity used, yet lower intensities combined with external heating to achieve the same temperature rise are shown to catalyze the reaction less effectively. Furthermore, measurements performed using equivalent external heating and an intensity too low to photocatalyze the reaction display no evidence of the reaction occurring. By providing high‐fidelity operando surface temperature measurements, this method offers a valuable tool for elucidating thermal contributions to plasmonic photocatalysis.

Variation in the thermal conductivity of a metal-coated tri-walled carbon nanotube (3WCNT), in the presence of vacancies, was studied using non-equilibrium molecular dynamics simulations. A Two-Temperature model was used to account for electronic contribution to heat transfer. For 3WCNT with 0.5% and 1% random vacancies, there was 76%, and 86% decrease in the thermal conductivity, respectively. In that order, an overall ~66% and ~140% increase in the thermal conductivity was recorded when 3 nm thick coating of metal (nickel) was deposited around the defective models. We have also explored the effects of tube specific and random vacancies on thermal conductivity of the 3WCNT. The changes in thermal conductivity have also been justified by the changes in vibrational density of states of the 3WCNT and the individual tubes. The results obtained can prove to be useful for countering the detrimental effects of vacancies in carbon nanotubes.

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