title: Instantaneous ballistic velocity of suspended Brownian nanocrystals measured by upconversion nanothermometry.
abstract: Brownian motion is one of the most fascinating phenomena in nature. Its conceptual implications have a profound impact in almost every field of science and even economics, from dissipative processes in thermodynamic systems, gene therapy in biomedical research, artificial motors and galaxy formation to the behaviour of stock prices. However, despite extensive experimental investigations, the basic microscopic knowledge of prototypical systems such as colloidal particles in a fluid is still far from being complete. This is particularly the case for the measurement of the particles' instantaneous velocities, elusive due to the rapid random movements on extremely short timescales. Here, we report the measurement of the instantaneous ballistic velocity of Brownian nanocrystals suspended in both aqueous and organic solvents. To achieve this, we develop a technique based on upconversion nanothermometry. We find that the population of excited electronic states in NaYF4:Yb/Er nanocrystals at thermal equilibrium can be used for temperature mapping of the nanofluid with great thermal sensitivity (1.15% K-1at 296 K) and a high spatial resolution (<1 μm). A distinct correlation between the heat flux in the nanofluid and the temporal evolution of Er3+emission allows us to measure the instantaneous velocity of nanocrystals with different sizes and shapes.
authors.affiliation: Department of Physics and CICECO, Aveiro Institute of Materials, Universidade de Aveiro, 3810-193 Aveiro, Portugal.
authors.forename: Carlos D S
authors.identifier: 0000-0001-9636-2628
authors.initials: CD
authors.lastname: Brites
authors.affiliation: Key Laboratory of Flexible Electronics &Institute of Advanced Materials, Jiangsu National Synergetic Innovation Center for Advanced Materials, Nanjing Tech University, Nanjing 211816, China.
authors.forename: Xiaoji
authors.initials: X
authors.lastname: Xie
authors.affiliation: Department of Physics and CICECO, Aveiro Institute of Materials, Universidade de Aveiro, 3810-193 Aveiro, Portugal.
authors.forename: Mengistie L
authors.identifier: 0000-0003-2516-9665
authors.initials: ML
authors.lastname: Debasu
authors.affiliation: Institute of Materials Research and Engineering, Agency for Science, Technology and Research, 2 Fusionopolis Way, 138634 Singapore, Singapore.
authors.forename: Xian
authors.initials: X
authors.lastname: Qin
authors.affiliation: Key Laboratory for Organic Electronics and Information Displays &Institute of Advanced Materials, Jiangsu National Synergetic Innovation Center for Advanced Materials, Nanjing University of Posts and Telecommunications, Nanjing 210023, China.
authors.forename: Runfeng
authors.initials: R
authors.lastname: Chen
authors.affiliation: Key Laboratory of Flexible Electronics &Institute of Advanced Materials, Jiangsu National Synergetic Innovation Center for Advanced Materials, Nanjing Tech University, Nanjing 211816, China.|Key Laboratory for Organic Electronics and Information Displays &Institute of Advanced Materials, Jiangsu National Synergetic Innovation Center for Advanced Materials, Nanjing University of Posts and Telecommunications, Nanjing 210023, China.
authors.forename: Wei
authors.initials: W
authors.lastname: Huang
authors.affiliation: Departament of Chemistry and CICECO, Universidade de Aveiro, 3810-193 Aveiro, Portugal.
authors.forename: João
authors.initials: J
authors.lastname: Rocha
authors.affiliation: Institute of Materials Research and Engineering, Agency for Science, Technology and Research, 2 Fusionopolis Way, 138634 Singapore, Singapore.|Department of Chemistry, National University of Singapore, 117543 Singapore, Singapore.
authors.forename: Xiaogang
authors.identifier: 0000-0003-2517-5790
authors.initials: X
authors.lastname: Liu
authors.affiliation: Department of Physics and CICECO, Aveiro Institute of Materials, Universidade de Aveiro, 3810-193 Aveiro, Portugal.
authors.forename: Luís D
authors.identifier: 0000-0003-4747-6535
authors.initials: LD
authors.lastname: Carlos