Green Thomas D, Yi Chongyue, Zeng Chenjie, Jin Rongchao, McGill Stephen, Knappenberger Kenneth L
Department of Chemistry and Biochemistry, Florida State University , Tallahassee, Florida 32306-4390, United States.
J Phys Chem A. 2014 Nov 13;118(45):10611-21. doi: 10.1021/jp505913j. Epub 2014 Sep 16.
Temperature-dependent photoluminescence of structurally precise Au25(SC8H9)18 and Au38(SC12H25)24 monolayer-protected cluster (MPC) nanoparticles were studied using energy-resolved, intensity-integrated, and time-resolved spectroscopy. Measurements were carried out at sample temperatures spanning the range from 4.5 to 200 K following electronic excitation using 3.1 eV pulsed lasers. The integrated PL intensity for Au25(SC8H9)18 increased sharply by 70% as the sample temperature was increased from 4.5 to 45 K. The PL intensity was statistically invariant for temperatures between 45 and 65 K but was quenched when the sample temperature was raised above 65 K. For both MPC samples, the global PL emission included several components. Each PL component exhibited an increase in emission energy when the sample temperature was increased from 4.5 to 40 K. This unexpected behavior may imply that MPCs in the 1 nm domain have negative expansion coefficients. Quantitative analysis of PL emission energies and peak widths obtained at sample temperatures greater than 45 K indicated MPC nonradiative relaxation dynamics are mediated by coupling to low-frequency vibrations associated with the ligand shell that passivated the nanoclusters, which accounted for the low emission yields at high sample temperatures. Contributions from two different vibrational modes were identified: Au(I)-S stretching (200 cm(-1)) and Au(0)-Au(I) stretching (90 cm(-1)). Analysis of each PL component revealed that the magnitude of electronic-vibration coupling was state-specific, and consistently larger for the high-energy portions of the PL spectra. The total integrated PL intensity of the Au25(SC8H9)18 MPC was correlated to the relative branching ratios of the emission components, which confirmed decreased emission for recombination channels associated with strong electron-vibration coupling and high emission yields for low emission energies at low temperature. The efficient low-energy emission was attributed to a charge-transfer PL transition. This conclusion was reached based on the strong correlation between temperature-dependent intensity-integrated and time-resolved emission measurements that revealed an ∼3.5-5.5 meV activation barrier to nonradiative decay. These findings suggest that nanoscale structure and composition can be modified to tailor the optical and mechanical properties and electronic relaxation dynamics of MPC nanostructures.
利用能量分辨、强度积分和时间分辨光谱，研究了结构精确的Au25(SC8H9)18和Au38(SC12H25)24单层保护簇（MPC）纳米颗粒的温度依赖光致发光。在使用3.1 eV脉冲激光进行电子激发后，在4.5至200 K的样品温度范围内进行测量。随着样品温度从4.5 K升高到45 K，Au25(SC8H9)18的积分PL强度急剧增加70%。在45至65 K之间，PL强度在统计上不变，但当样品温度升高到65 K以上时，PL强度猝灭。对于这两种MPC样品，整体PL发射包括几个成分。当样品温度从4.5 K升高到40 K时，每个PL成分的发射能量都增加。这种意外行为可能意味着1 nm范围内的MPC具有负膨胀系数。对样品温度高于45 K时获得的PL发射能量和峰宽进行定量分析表明，MPC的非辐射弛豫动力学是通过与钝化纳米团簇的配体壳层相关的低频振动耦合介导的，这解释了在高样品温度下的低发射产率。确定了两种不同振动模式的贡献：Au(I)-S拉伸（200 cm(-1)）和Au(0)-Au(I)拉伸（90 cm(-1)）。对每个PL成分的分析表明，电子-振动耦合的大小是状态特异性的，并且在PL光谱的高能部分始终更大。Au25(SC8H9)18 MPC的总积分PL强度与发射成分的相对分支比相关，这证实了与强电子-振动耦合相关的复合通道发射减少，以及低温下低发射能量的高发射产率。高效的低能发射归因于电荷转移PL跃迁。这一结论是基于温度依赖的强度积分和时间分辨发射测量之间的强相关性得出的，该相关性揭示了约3.5-5.5 meV的非辐射衰减激活势垒。这些发现表明，可以修改纳米级结构和组成，以调整MPC纳米结构的光学和机械性能以及电子弛豫动力学。
