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The Role of a Phonon Bottleneck in Relaxation Processes for Ln-Doped NaYF4 Nanocrystals.
The Journal of Physical Chemistry C ( IF 3.3 ) Pub Date : 2018-02-05 , DOI: 10.1021/acs.jpcc.7b11171 Jacobine J H A van Hest 1 , Gerhard A Blab 1 , Hans C Gerritsen 1 , Celso de Mello Donega 1 , Andries Meijerink 1
The Journal of Physical Chemistry C ( IF 3.3 ) Pub Date : 2018-02-05 , DOI: 10.1021/acs.jpcc.7b11171 Jacobine J H A van Hest 1 , Gerhard A Blab 1 , Hans C Gerritsen 1 , Celso de Mello Donega 1 , Andries Meijerink 1
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The localized inner 4f shell transitions of lanthanide ions are largely independent of the local surroundings. The luminescence properties of Ln3+ ions doped into nanocrystals (NCs) are therefore similar to those in bulk crystals. Quantum size effects, responsible for the unique size-dependent luminescence of semiconductor NCs, are generally assumed not to influence the optical properties of Ln3+-doped insulator NCs. However, phonon confinement effects have been reported to hamper relaxation between closely spaced Stark levels in Ln3+-doped NCs. At cryogenic temperatures emission and excitation from higher Stark levels was observed for Ln3+ ions in NCs only and were explained by a cutoff in the acoustic phonon spectrum. Relaxation would be inhibited as no resonant low energy (long wavelength) acoustic phonon modes can exist in nanometer sized crystals, and this prevents relaxation by direct phonon emission between closely spaced Stark levels. This phenomenon is known as a phonon bottleneck. Here, we investigate the role of phonon confinement in Ln-doped NCs. High resolution emission spectra at temperatures down to 2.2 K are reported for various Ln3+ ions (Er3+, Yb3+, Eu3+) doped into monodisperse 10 nm NaYF4 NCs and compared with spectra for bulk (microcrystalline) material. Contrary to previous reports, we find no evidence for phonon bottleneck effects in the emission spectra. Emission from closely spaced higher Stark levels is observed only at high excitation powers and is explained by laser heating. The present results indicate that previously reported effects in NCs may not be caused by phonon confinement.
中文翻译：
掺Ln的NaYF4纳米晶体的声子瓶颈在弛豫过程中的作用。
镧系元素离子的局部内部4f壳转变在很大程度上与局部环境无关。因此，掺杂到纳米晶体（NCs）中的Ln3 +离子的发光特性类似于体晶体中的发光特性。通常假定量子尺寸效应负责半导体NC的独特的尺寸依赖性发光，而不会影响掺杂Ln3 +的绝缘体NC的光学性能。然而，据报道，声子限制效应阻碍了Ln3 +掺杂NCs中紧密间隔的Stark水平之间的弛豫。在低温下，仅在NC中观察到了Ln3 +离子的较高Stark发射和激发，并且通过声子声子谱的截止来解释。弛豫将被抑制，因为在纳米尺寸的晶体中不会存在共振低能量（长波长）声子声子模，并且这将防止在紧密分布的Stark能级之间通过直接声子发射而引起的弛豫。这种现象被称为声子瓶颈。在这里，我们调查声子限制在掺Ln的NC中的作用。据报道，掺杂到单分散10 nm NaYF4 NC中的各种Ln3 +离子（Er3 +，Yb3 +，Eu3 +）在低至2.2 K的温度下具有高分辨率发射光谱，并与块状（微晶）材料的光谱进行了比较。与以前的报告相反，我们在发射光谱中没有发现声子瓶颈效应的证据。只有在高激发功率下才能观察到间隔较近的较高Stark发射的发射，并且可以通过激光加热来解释。
更新日期：2018-02-08
中文翻译：
掺Ln的NaYF4纳米晶体的声子瓶颈在弛豫过程中的作用。
镧系元素离子的局部内部4f壳转变在很大程度上与局部环境无关。因此，掺杂到纳米晶体（NCs）中的Ln3 +离子的发光特性类似于体晶体中的发光特性。通常假定量子尺寸效应负责半导体NC的独特的尺寸依赖性发光，而不会影响掺杂Ln3 +的绝缘体NC的光学性能。然而，据报道，声子限制效应阻碍了Ln3 +掺杂NCs中紧密间隔的Stark水平之间的弛豫。在低温下，仅在NC中观察到了Ln3 +离子的较高Stark发射和激发，并且通过声子声子谱的截止来解释。弛豫将被抑制，因为在纳米尺寸的晶体中不会存在共振低能量（长波长）声子声子模，并且这将防止在紧密分布的Stark能级之间通过直接声子发射而引起的弛豫。这种现象被称为声子瓶颈。在这里，我们调查声子限制在掺Ln的NC中的作用。据报道，掺杂到单分散10 nm NaYF4 NC中的各种Ln3 +离子（Er3 +，Yb3 +，Eu3 +）在低至2.2 K的温度下具有高分辨率发射光谱，并与块状（微晶）材料的光谱进行了比较。与以前的报告相反，我们在发射光谱中没有发现声子瓶颈效应的证据。只有在高激发功率下才能观察到间隔较近的较高Stark发射的发射，并且可以通过激光加热来解释。
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