title: Femtosecond Measurements Of Size-Dependent Spin Crossover In Fe(II)(pyz)Pt(CN)4 Nanocrystals.
abstract: We report a femtosecond time-resolved spectroscopic study of size-dependent dynamics in nanocrystals (NCs) of Fe(pyz)Pt(CN)4. We observe that smaller NCs (123 or 78 nm cross section and <25 nm thickness) exhibit signatures of spin crossover (SCO) with time constants of ∼5-10 ps whereas larger NCs with 375 nm cross section and 43 nm thickness exhibit a weaker SCO signature accompanied by strong spectral shifting on a ∼20 ps time scale. For the small NCs, the fast dynamics appear to result from thermal promotion of residual low-spin states to high-spin states following nonradiative decay, and the size dependence is postulated to arise from differing high-spin vs low-spin fractions in domains residing in strained surface regions. The SCO is less efficient in larger NCs owing to their larger size and hence lower residual LS/HS fractions. Our results suggest that size-dependent dynamics can be controlled by tuning surface energy in NCs with dimensions below ∼25 nm for use in energy harvesting, spin switching, and other applications.
authors.affiliation: JILA, National Institute of Standards and Technology (NIST), and University of Colorado at Boulder , 440 UCB, Boulder, Colorado 80309, United States.
authors.forename: D M
authors.initials: DM
authors.lastname: Sagar
authors.affiliation: National Renewable Energy Laboratory , 15013 Denver West Parkway, Golden, Colorado 80401, United States.
authors.forename: Frederick G
authors.initials: FG
authors.lastname: Baddour
authors.affiliation: JILA, National Institute of Standards and Technology (NIST), and University of Colorado at Boulder , 440 UCB, Boulder, Colorado 80309, United States.
authors.forename: Patrick
authors.initials: P
authors.lastname: Konold
authors.affiliation: NIST , 325 Broadway, Boulder, Colorado 80305, United States.
authors.forename: Joel
authors.initials: J
authors.lastname: Ullom
authors.affiliation: National Renewable Energy Laboratory , 15013 Denver West Parkway, Golden, Colorado 80401, United States.
authors.forename: Daniel A
authors.initials: DA
authors.lastname: Ruddy
authors.affiliation: National Renewable Energy Laboratory , 15013 Denver West Parkway, Golden, Colorado 80401, United States.
authors.forename: Justin C
authors.initials: JC
authors.lastname: Johnson
authors.affiliation: JILA, National Institute of Standards and Technology (NIST), and University of Colorado at Boulder , 440 UCB, Boulder, Colorado 80309, United States.|Department of Chemistry and Biochemistry, University of Colorado at Boulder , 215 UCB, Boulder, Colorado 80309, United States.
authors.forename: Ralph
authors.initials: R
authors.lastname: Jimenez
keywords: High spin; Low spin; femtosecond; nanocrystals; spin-crossover; time-resolved spectroscopy; transient absorption