NMR-based gap behavior related to the quantum size effect

Abstract

We conducted 195 Pt-nuclear magnetic resonance measurements on various-diameter Pt nanoparticles coated with polyvinylpyrrolidone in order to detect the quantum size effect and the discrete energy levels in the electron density of states, both of which were predicted by Kubo more than 50 years ago. We succeeded in separating the signals arising from the surface and interior regions and found that the nuclear spin-lattice relaxation rates in both regions show the metallic behavior at high temperatures. Surprisingly, the magnetic fluctuations in both regions exhibited anomalous behavior below the same temperature T * , which points to a clear size dependence and is well scaled with δ Kubo . These results suggest that a size-tunable metal-insulator transition occurs in the Pt nanoparticles as a result of the Kubo effect.

Magnetic-Field Dependence of Novel Gap Behavior Related to the Quantum-Size Effect

“…5,6) We have previously investigated the electronic properties of Pt nanoparticles with nuclear magnetic resonance (NMR) measurements. 7) In our measurements, the NMR signals corresponding to the surface could be distinguished from those corresponding to the interior regions of the nanoparticles. This was possible because of the advantage of the large Pt Knight shift owing to the large density of states of d electrons.…”

“…The systematic particle size dependence suggests that k B T * would correspond to the "Kubo" discrete-energy gap. 7) The H dependence of T * is crucial to support the above hypothesis. If the above statements are true, T * will be affected by H via the Zeeman effect.…”

“…In our previous study, 7) we focused on the nanoparticles of d-electron metals with a large DOS at the Fermi level, which is useful to distinguish the QSE from the surface effect. In addition, the Pt nucleus is suitable for NMR measurements; 9) the gyromagnetic ratio of 195 Pt is large (γ n = 9.153 MHz/T) with a nuclear spin of I = 1/2, and thus, the nuclear quadrupole interaction is absent in the Pt nucleus.…”

“…The sample synthesis conditions are described in the previous paper. 7) For the NMR measurements, we used 500-mg-weight samples (Pt nanoparticles coated with Polyvinylpyrrolidone). The NMR measurements were performed in a wide magnetic field ranging from ∼ 2.8 T (25.35 MHz) to ∼ 23.3 T (212.03 MHz).…”

“…Therefore, the NMR signals arising from the surface and interior regions were determined based on the Knight-shift values, as done in the previous study. 7) The temperature dependence of 1/T 1 on the surface in the interior regions of the nanoparticles in various magnetic fields is shown in Fig. 1.…”