本論文中我們使用超快光激發-兆赫波探測(Optical-pump THz-probe)技術觀察化學氣相沉積法成長的錯位雙層石墨烯其超快載子動力學，並藉由液態閘極給予石墨烯偏壓，使其費米能階改變，觀察不同費米能階下光載子的弛豫行為。
在最早對磊晶成長的石墨烯進行研究，觀察光激發後穿透率變化為負值即光電導率變化為正值，為似半導體行為，而直到2013~2014年才有對化學氣相沉積法成長的單層石墨烯進行研究，並出現兩種不同的看法一是觀察光激發後穿透率變化為在從遠離接近至迪拉克點出現穿透率變化從正值變負值，二是光激發後穿透率變化恆為正值，恆為似金屬行為，但是在接近迪拉克點時訊號接近於零，而本文中在對化學氣相沉積法成長的錯位雙層石墨烯進行研究後，觀察到與第二種看法的單層石墨烯有相似的結果與趨勢，其原因是因為錯位雙層石墨烯的層與層之間雖然是凡德瓦力，但是依然可以傳導電子，而唯一可能會讓弛豫行為產生差別的是，經由拉曼光譜所得知，錯位雙層石墨烯的聲子色散模式會有些改變，但是改變過小，使得最終對光載子的弛豫行為影響不明顯，因此最後我們使用分析單層石墨烯的方式，解釋錯位雙層石墨烯裡光載子的弛豫現象。

In this thesis, we use optical-pump THz-probe spectroscopy to measure the ultrafast relaxation dynamics of quasi-particle in the misoriented bilayer graphene, which was fabricated by chemical vapor deposition (CVD), and then we use liquid gate to change the Fermi level of the sample for observing carrier relaxation dynamic. In early, the research about the relaxation dynamics on graphene which grown on SiC substrate was published, and it showed the negative pump induced differential transmission (PIDT), that is, a transient positive conductivity after photoexcitation. In 2013-2014, there were the different results on CVD monolayer graphene which showed the positive PIDT, and there are two kind of opinions when the Fermi level near the Dirac point. First showed the PIDT will change sign from positive to negative when near the Dirac point. Second showed the PIDT will always positive in all Fermi level and the signal will vanish when near the Dirac point, finally, the behavior of misoriented bilayer graphene is quasi-similar with the second one. Our sample differs from monolayer graphene. Electrons can transmit between two layers, and misoriented bilayer graphene have different optical phonon dispersion relation by Raman spectroscopy. However, this different is too small to have a significant impact on the carrier relaxation dynamic. The feature of response is similar to that of monolayer, therefore, we explain our experiment results by using the analysis method in monolayer graphene.

[1] J.C.Slonczewski and P.R. Weiss, Band Structure of Graphite, Phys. Rev. 109, 272(1958)
[2] K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, M. I. Katsnelson, I. V. Grigorieva, S. V. Dubonos, and A. A. Firsov, Two-dimensional gas of massless Dirac fermions in graphene, Nature 438, 197 (2005)
[3] K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang, S. V. Dubonos, I. V. Grigorieva, and A. A. Firsov, Electric Field Effect in Atomically Thin Carbon Films, Science 306, 666 (2004)
[4] Y. B. Zhang, Y. W. Tan, H. L. Stormer, and P. Kim, Experimental observation of the quantum Hall effect and Berry's phase in graphene, Nature 438, 201 (2005)
[5] J.M. Dawlaty, Shriram Shivaraman, Mvs Chandrashekhar, Farhan Rana and Michael G. Spencer, Measurement of ultrafast carrier dynamics in epitaxial graphene ,Appl. Phys. Lett. 92, 042116 (2008)
[6] P.A. George, Jared Strait, Jahan Dawlaty, Shriram Shivaraman, Mvs Chandrashekhar, Farhan Rana and Michael G. Spencer, Ultrafast Optical-Pump Terahertz-Probe Spectroscopy of the Carrier Relaxation and Recombination Dynamics in Epitaxial Graphene ,Nano Lett. 8, 4248(2008)
[7] H. Choi, F. Borondics, D. A. Siegel, S. Y. Zhou,M. C. Martin, A. Lanzara, and R. A. Kaindl, Broadband electromagnetic response and ultrafast dynamics of few-layer epitaxial graphene , Appl. Phys. Lett. 94,172102 (2009)
[8] K. J. Tielrooij, J. C.W. Song, S. A. Jensen, A. Centeno, A. Pesquera, A. Zurutuza Elorza, M. Bonn, L. S. Levitov and F. H. L. Koppens, Photoexcitation cascade and multiple hot-carrier generation in graphene ,Nature Phys. 9, 248 (2013)
[9] G. Jnawali, Yi Rao, Hugen Yan and Tony F. Heinz, Observation of a Transient Decrease in Terahertz Conductivity of Single-Layer Graphene Induced by Ultrafast Optical Excitation, Nano Lett. 13,524 (2013)
[10] A. J. Frenzel, C. H. Lui, Y. C. Shin, J. Kong, and N. Gedik, Semiconducting- to-Metallic Photoconductivity Crossover and Temperature-Dependent Drude Weight in Graphene, Phys. Rev. Lett. 113, 056602 (2014)
[11] S.F. Shi, T.T. Tang, B. Zeng, L. Ju, Q. Zhou, A. Zettl, and F. Wang, Controlling Graphene Ultrafast Hot Carrier Response from Metal-like to Semiconductor-like by Electrostatic Gating, Nano Lett.14, 1578 (2014)
[12] Kuan-chun Lin, Ultrafast Carrier Dynamics of Chemical Vapor Deposited Graphene, Nthu(2013)
[13] P. R. Wallace, The Band Theory of Graphite ,Phys. Rev. 71, 622(1947)
[14] L.M. Malard, M.A. Pimenta, G. Dresselhaus, M.S. Dresselhaus, Raman spectroscopy in graphene ,Phys. Rep. 473, 51 (2009)
[15] F. Rana, Paul A. George, Jared H. Strait, Jahan Dawlaty, Shriram Shivaraman, Mvs Chandrashekhar, Michael G. Spencer, Carrier recombination and generation rates for intravalley and intervalley phonon scattering in graphene, Phys. Rev. B 79, 115447 (2009)
[16] 劉映吾, The Fabrication and Characterization of Chemical Vapor Deposited Graphene, Nthu (2013)
[17] K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang, S. V. Dubonos, I. V. Grigorieva, A. A. Firsov, Electric Field Effect in Atomically Thin Carbon Films ,Science 306, 666 (2004)
[18] Xiaolin Li, Guangyu Zhang, Xuedong Bai, Xiaoming Sun, Xinran Wang, Enge Wang & Hongjie Dai, Highly conducting graphene sheets and Langmuir–Blodgett films, Nature Nanotech. 3, 538 (2008)
[19] J. Hass, W A de Heer and E H Conrad, The growth and morphology of epitaxial multilayer graphene, J. Phys.: Cond. Mat. 20, 323202 (2008)
[20] A. Gupta, G. Chen, P. Joshi, S. Tadigadapa, and P.C. Eklund, Raman scattering from high-frequency phonons in supported n-graphene layer films , Nano Lett.6, 2667 (2006)
[21] Zhenhua Ni, Yingying Wang, Ting Yu, Yumeng You, and Zexiang Shen, Reduction of Fermi velocity in folded graphene observed by resonance Raman spectroscopy, Phys. Rev. B 77, 235403 (2008)
[22] M.-Y. Li, C.-C. Tang, D. C. Ling, L. J. Li, C. C. Chi, and J. C. Chen, Charged impurity-induced scatterings in chemical vapor deposited grapheme, J. Appl. Phys. 114, 233703 (2013)