Observation of Ultrafast Carrier Dynamics and Phonon Relaxation of Graphene from the Deep-Ultraviolet to the Visible Region
Observation of Ultrafast Carrier Dynamics and Phonon Relaxation of Graphene from the Deep-Ultraviolet to the Visible Region
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DOI：
10.1021/jp4072197
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发表时间：
2014-03-27
影响因子：
3.7
通讯作者：
Mueller, Andreas
中科院分区：
文献类型：
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作者：
Oum, Kawon;Lenzer, Thomas;Mueller, Andreas
我们研究了石英衬底上CVD生长的单分子膜和9层石墨烯的超快载流子动力学和声子弛豫。激发波长分别为400和800 nm。在260-640 nm(1.94-4.77 eV)范围内探测了光密度的归一化变化，深入到石墨烯的Fano共振区域，这是以前在飞秒宽带泵浦-探测实验中没有研究过的。测得时间常数为160ps和4ps，分别归属于载流子-光学声子散射和较慢的声子弛豫过程。400 nm激发的早期载流子分布明显高于800 nm激发的载流子分布。在300 nm以下观察到明显的光谱漂白特征。单层石墨烯的时间常数为35ps，九层石墨烯的时间常数为120和970ps。在约0.5ps后，300 nm以上的微弱暂态吸收特征也表现出相同的动力学特征。从石墨烯到石英衬底的界面热流是缓慢的动力学过程。漂白和吸收特性可以用一个简单的模型很好地描述，该模型假定Fano共振发生红移。这种红移随着石墨烯的逐渐冷却而消失。因此，我们认为红移是由于晶格加热引起的能带分离的收缩所致。
We investigated the ultrafast carrier dynamics and phonon relaxation of CVD-grown monolayer and 9-layer graphene on a quartz substrate. Excitation was performed at 400 and 800 nm. The normalized change in optical density Delta OD was probed over the range 260-640 nm (1.94-4.77 eV), reaching down into the region of graphene's Fano resonance, previously not investigated in femtosecond broadband pump-probe experiments. Time constants of 160 fs and 4 Ps were found and assigned to carrier-optical phonon scattering and slower phonon relaxation processes, respectively. The carrier distribution at early times was clearly hotter for 400 nm excitation than for 800 nm excitation. A pronounced spectral bleach feature was observed below 300 nm. It immediately formed after photoexcitation and recovered slowly, with a time constant of 35 ps for monolayer and time constants of 120 and 970 ps for 9-layer graphene. The same dynamics were found for weak transient absorption features above 300 nm, which emerged after ca. 0.5 Ps. The slow dynamics were assigned to interfacial heat flow from graphene to the quartz substrate. The bleach and absorption features were well described by a simple model assuming a red-shift of the Fano resonance. This red-shift disappeared with progressive cooling of graphene. We therefore suggest that the red-shift is induced by shrinking of the band separation due to lattice heating.