Evidence for formation of multi-quantum dots in hydrogenated graphene.

We report the experimental evidence for the formation of multi-quantum dots in a hydrogenated single-layer graphene flake. The existence of multi-quantum dots is supported by the low-temperature measurements on a field effect transistor structure device. The resulting Coulomb blockade diamonds shown in the color scale plot together with the number of Coulomb peaks exhibit the characteristics of the so-called 'stochastic Coulomb blockade'. A possible explanation for the formation of the multi-quantum dots, which is not observed in pristine graphene to date, was attributed to the impurities and defects unintentionally decorated on a single-layer graphene flake which was not treated with the thermal annealing process. Graphene multi-quantum dots developed around impurities and defect sites during the hydrogen plasma exposure process.RIGHTS : This article is licensed under the BioMed Central licence at http://www.biomedcentral.com/about/license which is similar to the 'Creative Commons Attribution Licence'. In brief you may : copy, distribute, and display the work; make derivative works; or make commercial use of the work - under the following conditions: the original author must be given credit; for any reuse or distribution, it must be made clear to others what the license terms of this work are

Experimental evidence for direct insulator-quantum Hall transition in multi-layer graphene

We have performed magnetotransport measurements on a multi-layer graphene flake. At the crossing magnetic field Bc, an approximately temperature-independent point in the measured longitudinal resistivity, which is ascribed to the direct insulator-quantum Hall (I-QH) transition, is observed. By analyzing the amplitudes of the magnetoresistivity oscillations, we are able to measure the quantum mobility of our device. It is found that at the direct I-QH transition, the product of the quantum mobility and is about 0.37 which is considerably smaller than 1. In contrast, at Bc, the longitudinal resistivity is close to the Hall resistivity, i.e., the product of the classical mobility and the crossing field is about 1. Therefore our results suggest that different mobilities need to be introduced for the direct I-QH transition observed in multi-layered graphene. Combined with existing experimental results obtained in various material systems, our data obtained on graphene suggest that the direct I-QH transition is a universal effect in 2D.Comment: 4 figure

High Current-Induced Electron Redistribution in a CVD-Grown Graphene Wide Constriction

Investigating the charge transport behavior in one-dimensional quantum confined system such as the localized states and interference effects due to the nanoscale grain boundaries and merged domains in wide chemical vapor deposition graphene constriction is highly desirable since it would help to realize industrial graphene-based electronic device applications. Our data suggests a crossover from interference coherent transport to carriers flushing into grain boundaries and merged domains when increasing the current. Moreover, many-body fermionic carriers with disordered system in our case can be statistically described by mean-field Gross-Pitaevskii equation via a single wave function by means of the quantum hydrodynamic approximation. The novel numerical simulation method supports the experimental results and suggests that the extreme high barrier potential regions on graphene from the grain boundaries and merged domains can be strongly affected by additional hot charges. Such interesting results could pave the way for quantum transport device by supplying additional hot current to flood into the grain boundaries and merged domains in one-dimensional quantum confined CVD graphene, a great advantage for developing graphene-based coherent electronic devices

Charge Trapping in Monolayer and Multilayer Epitaxial Graphene

We have studied the carrier densities n of multilayer and monolayer epitaxial graphene devices over a wide range of temperatures T. It is found that, in the high temperature regime (typically T ≥ 200 K), ln⁡(n) shows a linear dependence of 1/T, showing activated behavior. Such results yield activation energies ΔE for charge trapping in epitaxial graphene ranging from 196 meV to 34 meV. We find that ΔE decreases with increasing mobility. Vacuum annealing experiments suggest that both adsorbates on EG and the SiC/graphene interface play a role in charge trapping in EG devices

Magnetotransport in an aluminum thin film on a GaAs substrate grown by molecular beam epitaxy

Magnetotransport measurements are performed on an aluminum thin film grown on a GaAs substrate. A crossover from electron- to hole-dominant transport can be inferred from both longitudinal resistivity and Hall resistivity with increasing the perpendicular magnetic field B. Also, phenomena of localization effects can be seen at low B. By analyzing the zero-field resistivity as a function of temperature T, we show the importance of surface scattering in such a nanoscale film

Electronic properties of disordered graphene

二維石墨烯，因擁有較高的載子遷移率和高電流密度特性，並且其元件傳輸特性在10奈米以下，相較於矽，仍保持著優異的傳輸特性，因此被視為取代矽並延伸莫爾定律之有前景候選材料。對於大面積的工業量產而言，石墨烯固有的瑕疵，例如：缺陷、無序性、雜質是在製造碳化半導體元件時不可避免的因素。因此為了實現製出石墨烯化的半導體元件以致於延伸摩爾定律。關於無序的石墨烯系統，更加了解其基礎傳輸物理特性是一件重要且急迫的議題。 此篇論文回顧一系列在無序石墨烯系統-極高無序性的氫化石墨烯和適當無序性的多層石墨烯之載子傳輸實驗，其中展示出許多的量子傳輸行為，這樣的研究將能為了延伸莫爾定律，朝向新穎的石墨烯化元件提供一條新的發展方向。 第一個實驗利用修正過的電阻曲線引出的分析法(RCDA)在電性與磁性的傳輸中展示出 Efros-Shklovskii變程躍遷(ES-VRH)在氫化的石墨烯系統中是主要的傳輸機制。更進一步，二維多重量子點可以在不執行膠帶剝離法之熱退火進行氫離子曝露過程中形成氫化石墨烯。因此在極高無序石墨烯中，二維狀態是極其侷限的並且庫侖作用力在此系統中扮演一個重要的角色。 第二個實驗在多層石墨烯中縱向電阻率展現出一個趨近溫度不相依的相交點，其歸因於直接從絕緣態到量子霍爾態的相變和在多層石墨烯和單層石墨烯觀測到磁電導漲落。由於在許多其它二維材料中也曾觀測到此現象，因此我們強烈建議在石墨烯系統中從直接絕緣態到量子霍爾態的現象是一種二維系統的普遍現象。另一方面，在多層石墨烯，低溫的退相干飽和時間比單層的石墨烯少一個數量級，並且多層石墨烯低溫的退相干飽和時間的初始溫度比單層石墨烯還低，這在介觀傳輸系統中是一個不尋常的效應。我們建議多層石墨烯不只提供合適的無序特性來增加局限效應，也提供較好的傳輸通道來避免基底雜質和分子的散射，這樣的特性導致在介觀系統的不尋常現象。 最後的實驗引進多層石墨烯局限和載子熱效應，其中弱局域和普遍電導漲落效應共同存在和載子溫度和電流成性線關性在多層石墨烯系統。在多層石墨烯中，弱局域與普遍電導漲落效應的相干長度的不一致可能是來自於存在的多層石墨烯造成，它會增加無序性和局域化的情況。另一方面，迪拉克費米子的溫度TDF是正比於所輸入的電流，意指α值趨近於1。因此意指只有很小的迪拉克費米子與聲子之散射，這對於奈米元件的應用是一個很大的優勢。In order to extend Moore’s law, two-dimensional graphene, which exists much better carrier mobility, high current density and great transport performance rather than silicon when the device is below 10 nm, is a promising candidate to replace silicon. For large-scale industrial productions, the imperfections inherent in graphene such as defects, disorder and impurities, are unavoidable factors during fabricating carbon-based semiconductor devices. Consequently, it is an important and immediate issue to further understand the fundamental transport physics in disordered graphene systems in order to realize graphene-based semiconductor devices for extending Moore’s law. This thesis describes a series of experiments on carriers transport through disordered graphene systems – hydrogenated graphene in highly disordered property and multi-layer graphene in suitably disordered property, revealing different quantum transport behaviors in disordered graphene systems so as to pave the way towards novel graphene-based devices for extending Moore’s law. The first experiment presents Efros-Shklovskii variable range hopping is the principal transport mechanism in the hydrogenated graphene system by using the modified resistance curve derivative analysis method in electric and magneto transport. Moreover, two-dimensional multi-quantum dots can be formed on hydrogenated graphene without executing post-exfoliation thermal annealing during carrying on hydrogen plasma exposure process. Therefore, two-dimensional states in highly disordered graphene are strongly localized and Coulomb interactions play key roles. The second experiment shows a nearly temperature-independent point in the measured longitudinal resistivity ρ_xx, which is ascribed to the direct insulator-quantum Hall transition, in multi-layer graphene and magneto-conductance fluctuations in multi-layer and single-layer graphene. According to the experimental results on various two-dimensional materials, the direct insulator-quantum Hall transition observed in a graphene-based system strongly suggests that it is a universal effect in two-dimensional systems. Moreover, the low-temperature saturation of dephasing time in multi-layer graphene is one order of magnitude shorter than that in single-layer graphene, and the onset temperature of the low-temperature saturation of dephasing time in multi-layer graphene is lower than that in single-layer graphene, suggestive an uncommon effect in mescroscopic transport systems. We suggest that the multi-layer graphene not only provide suitably disordered property so as to enhance localized condition, but also provide some better transport channels by avoiding substrate impurities and molecules scattering, resulting in an uncommon phenomenon in mescroscopic systems. The last experiment introduces the localization and heating effect in multi-layer graphene, revealing weak localization and universal conductance fluctuations effect coexisted and the linear relation was between current and carrier temperature in the multi-layer graphene system. The disagreement of the phase coherence lengths between weak localization and universal conductance fluctuations effect in the multi-layer graphene is possibly due to the existence of multi-layer graphene which enhances the disordered property so as to improve the localization condition. Moreover, the Dirac fermion temperature TDF is proportional to driving current I, which suggests α ~ 1. Therefore, there is little Dirac fermion-phonon scattering, a great advantage for applications in nano-electronics

Magnetoresistance of Ultralow-Hole-Density Monolayer Epitaxial Graphene Grown on SiC

Silicon carbide (SiC) has already found useful applications in high-power electronic devices and light-emitting diodes (LEDs). Interestingly, SiC is a suitable substrate for growing monolayer epitaxial graphene and GaN-based devices. Therefore, it provides the opportunity for integration of high-power devices, LEDs, atomically thin electronics, and high-frequency devices, all of which can be prepared on the same SiC substrate. In this paper, we concentrate on detailed measurements on ultralow-density p-type monolayer epitaxial graphene, which has yet to be extensively studied. The measured resistivity ρxx shows insulating behavior in the sense that ρxx decreases with increasing temperature T over a wide range of T (1.5 K ≤ T ≤ 300 K). The crossover from negative magnetoresistivity (MR) to positive magnetoresistivity at T = 40 K in the low-field regime is ascribed to a transition from low-T quantum transport to high-T classical transport. For T ≥ 120 K, the measured positive MR ratio [ρxx(B) − ρxx(B = 0)]/ρxx(B = 0) at B = 2 T decreases with increasing T, but the positive MR persists up to room temperature. Our experimental results suggest that the large MR ratio (~100% at B = 9 T) is an intrinsic property of ultralow-charge-density graphene, regardless of the carrier type. This effect may find applications in magnetic sensors and magnetoresistance devices