| 摘要: | 石墨烯中與音頻 (acoustical)聲子相關的問題包括: 1)高溫和低溫狀態下的音頻聲子散射的不同特性，2)與摻雜濃度相關的布拉赫-格魯奈生(Bloch-Gruneisen)溫度，3)在Bloch-Gruneisen溫度之下，電子-平面聲子交互作用因受到聲子動量短缺的限制所產生的導電變化。在過去的十年中，第一和第二個問題吸引了許多理論和實驗研究，而第三個問題仍未有適當研究。值得一提的是，充分理解石墨烯中的這些問題不僅對於基礎理解很重要，而且對於設計石墨烯相關的元件如光學檢測器,輻射熱計，冷卻通道,和超碰撞等應用。
在這篇論文中，我們系統性的研究了這三個問題。關於第一個問題，我們嚴謹的推導了在任何有限溫度和摻雜下的非彈性和半彈性電子-聲子散射率。此推導證明了前人推導之高溫散射速率的正確性並校正了低溫散射率。另外在整個溫度範圍內，前人以經驗公式描述的散射率與我們的嚴謹理論所推導散射率相當接近,但是在低溫區域嚴重高估。作為測試平台，我們推導的散射率非常適合用來驗證文獻中的實驗數據。
對於第二個問題，我們發現前人討論的與摻雜相關的Bloch-Gruneisen溫度有許多待商榷之處。它們的值比正確值小了2到2.5倍。而且，我們推導出包含各種機制的總Bloch-Gruneisen溫度。使用我們的散射率來分析文獻中可用的實驗數據發現:用實驗數據推斷出的總Bloch-Gruneisen溫度與我們的理論預測值完全一致。此外，我們並指出了前人的理論和實驗工作的一些關鍵錯誤和不一致之處。我們的新結果質疑了許多前人有關石墨烯中摻雜相關的Bloch-Gruneisen溫度的理論研究。
最後，據我所知自1930年布拉赫（F. Bloch）和1933年E. Gruneisen的兩部作品以來，我們首次發現了在石墨烯中電子-聲子散射在Bloch-Grrneisen溫度下的確會受到聲子動量短缺的影響。在應用方面，我們用非彈性的散射率研究了在照光和不照光的情況下，p型石墨烯/ MoS2異質結構中在室溫(300 K)及柵極電壓下轉移電流的變化。;The in-plane acoustic phonon-related issues in graphene include the in-plane acoustic phonon
scatterings in the whole temperature range (from low- to high-temperature regime), the dopingdependent
Bloch-Gruneisen temperatures, and the effect of shortages of in-plane acoustic phonon
momenta to scatter off electrons at Bloch-Gruneisen temperatures. While the first and second
issues have been attracting a lot of theoretical and experimental researches during the last
decade, the third issue remains unexplored. It is worth mentioning that fully comprehending
these issues in graphene is not only important for fundamental understanding but also for designing
graphene-based devices such as optical detectors, bolometers, cooling pathways, and
supercollisions in graphene.
In this thesis, I systematically investigate the three issues in unprecedented details. Regarding
the first issue, the inelastic and semi-inelastic scattering rates at any finite temperature and
doping are derived rigorously, from which the high-temperature scattering rate is reproduced
and the low-temperature scattering rate is corrected. In addition, the ansatz scattering rate
manifests its asymptotic behavior to our scattering rates for the whole temperature range; especially,
the overestimation becomes greater in the low-temperature region. As a test bed, our
scattering rates well fit the available experimental data in the literature.
For the second issue, it turns out that the state-of-the-art definitions of the doping-dependent
Bloch-Gruneisen temperatures need to be revised. Their values should be about 2 ?? 2.5 times
smaller. Moreover, the total doping-dependent Bloch-Gruneisen temperatures emerge. Using
our scattering rates to analyze the available experimental data in the literature, the experimentally
inferred values of the total doping-dependent Bloch-Gruneisen temperatures completely
agree with our theoretically predicted values. Additionally, critical mistakes and inconsistencies
in some theoretical and experimental works are also pointed out. Furthermore, our new
results question many theoretical researches of formulations relating to the doping-dependent
Bloch-Gruneisen temperatures in graphene.
Finally, the last but not the least, as far as I have known since the two works by F. Bloch in
1930 and E. Gruneisen in 1933, shortages of acoustic phonon momenta to scatter off electrons
at doping-dependent Bloch-Gruneisen temperatures are observed for the first time. As an application,
we have used our scattering rates to study transfer current in p-type graphene/MoS2
heterostructures under a wide range of applied gate voltage at 300 K without and with optical
pumping. |
