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Nanocrystal Shape and Nano Junction Effects on Electron Transport in Nanocrystal-Assembled Bulks

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Nanostructured materials are materials with microstructures in the length scale of several nanometers, several tens of nanometers, or a hundred nanometers. Nanostructured materials can be made by physical methods of mechanical pressing or distortion. They can alternatively be made by self-assembly of chemically synthesized nanocrystals or nanoparticles. Nanocrystal-assembled bulks are also named quantum dot superlattices.1,2Electron transport in nanostructured materials have been investigated for decades. Several mechanisms, including Mott’s variable range hopping, nearest neighbor hopping, Efros-Shklovskii hopping, thermal activation, and fluctuation-induced tunneling conduction, have ever been used to describe the electron transport behavior in nanostructured materials.

Our questions as well as our goals are how we can choose a theoretical model based on the microstructure of nanostructured materials.

In our newly published paper,3 we discovered the importance of the nanocrystal shape and the nanojunction between nanocrystals in the determination of the electron transport in nanostructured materials. This is the first time that the transport mechanism can be explained in accordance with the microstructure of the nanostructured materials.

Professor Ping Sheng’s model of fluctuation-induced tunneling conduction is used to describe electron transport in our PbSe octahedral nanocrystal-assembled bulk. It is noted that an ultra-small area of tunneling junction between octahedral nanocystals results in such an electron transport behavior. The concept of electron transport can be considered as a point discharge effect of thermally excited electrons on the small tunneling junction area.

Ref. 1. O. L. Lazarenkova and A. A. Balandin, Phys. Rev. B, 2002, 66, 245319.

Ref. 2. Y. Bao, A. A. Balandin, J. L. Liu, J. Liu, and Y. H. Xie, Appl. Phys. Lett., 2004, 84, 3355.

Ref. 3. Shao-Chien Chiu, Jia-Sin Jhang, Yen-Fu Lin, Shih-Ying Hsu, Jiye Fang, and Wen-Bin Jian,* “Nanocrystal Shape and Nano Junction Effects on Electron Transport in Nanocrystal-Assembled Bulks”, Nanoscale 5, 8555-8559 (2013).

Probing into the metal-graphene interface by electron transport measurements

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The quality of metal contact on graphene affects the performance of electronic devices based on it. The contact effects shall be considered from both macroscopic and microscopic viewpoints. In 2008, the macroscopic viewpoint on the metal-graphene interface has been discussed from theoretical calculations1 and experimental determinations.2 It is understood that the metal contact causes a dipole layer in and a potential step across the interface,  and a shift of Fermi energy in graphene. On the other hand, the microscopic viewpoint which probes into the effect due to the roughness of the metal-graphene interface cannot be disregarded. In our new report,3 we applied the electron transport measurement to probe the microstructure of the metal-graphene interface and discovered the best model – Prof. Ping Sheng’s model of the fluctuation induced tunneling conduction (FITC) – to describe it. The model was ever used to describe the contact resistance of the metal to the carbon nanotube4 while the detail of the interface microstructure had not been discussed yet. We got the model parameters from fitting our data to the FITC model and we gave a profile of the microstructure in the metal-graphene interface.

Ref. 1. G. Giovannetti, P. A. Khomyakov, G. Brocks, V. M. Karpan, J. van den Brink, and P. J. Kelly, “Doping Graphene with Metal Contacts”, Phys. Rev. Lett. 101, 026803 (2008).

Ref. 2. Eduardo J. H. Lee, Kannan Balasubramanian, Ralf Thomas Weitz, Marko Burghard & Klaus Kern, “Contact and edge effects in graphene devices”, Nat. Nanotechnol. 3, 486 (2008).

Ref. 3. Yen-Fu Lin,* Sheng-Tsung Wang, Chia-Chen Pao, Ya-Chi Lee, Cheng-Chieh Lai, Chung-Kuan Lin, Shih-Ying Hsu, and Wen-Bin Jian,* “Probing into the metal-graphene interface by electron transport measurements”, Appl. Phys. Lett. 102, 033107 (2013).

Ref. 4. M. Salvato, M. Cirillo, M. Lucci, S. Orlanducci, I. Ottaviani, M. L. Terranova, and F. Toschi, “Charge Transport and Tunneling in Single-Walled Carbon Nanotube Bundles”, Phys. Rev. Lett. 101, 246804 (2008).

Direct probing of density of states of reduced graphene oxides in a wide voltage range by tunneling junction

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Graphene density of state near the Dirac point (in the range of several tens of meV) has been probed by using scanning tunneling spectroscopy and reported in the literature.1 However, the density of state, showing whole band structure, has not been experimentally determined yet. In our new paper,2 we report the fabrication of tunneling junctions on reduced graphene oxides and the measurement of differential conductance (dI/dV) in a wide energy range of several eV. The differential conductance exhibits the information of the density of state (see the black curve in the figure). The band edge is near 3 eV above/below the Dirac point and the experimental data are in line with the theoretical calculation (the green line) in consideration of both sigma and pi electron bands.

Ref. 1. Yuanbo Zhang, Victor W. Brar, Feng Wang, Caglar Girit, Yossi Yayon, Melissa Panlasigui, Alex Zettl & Michael F. Crommie, “Giant phonon-induced conductance in scanning tunnelling spectroscopy of gate-tunable graphene”, Nat. Phys. 4, 627 – 630 (2008)

Ref. 2. Sheng-Tsung Wang, Yen-Fu Lin, Ya-Chi Li, Pei-Ching Yeh, Shiow-Jing Tang, Baruch Rosenstein, Tai-Hsin Hsu, Xufeng Zhou, Zhaoping Liu, Minn-Tsong Lin, and Wen-Bin Jian, “Direct probing of density of states of reduced graphene oxides in a wide voltage range by tunneling junction”, Appl. Phys. Lett. 101, 183110 (2012).

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