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interview-professor-zhixian-zhou-wayne-univDr. Zhou is Assistant Professor at Wayne State University Physics Department. His research is centered on investigating the electrical transport properties of nanomaterials and devices. His research activities consist of three separate but interrelated programs: investigations on suspended graphene nanoribbons, investigations on two dimensional materials beyond graphene such as MoS2 and TiS2, and investigations on the application of graphene in chemical & biological sensing and cathode materials.Dr. Zhou has been a Park XE-70 AFM user since 2008.


Q: What is the field of your research?

My general field of research is nano materials and nano electronics. Basically, we fabricate devices out of graphene, and other quasi two-dimensional crystals of transition metal compounds such as molybdenum disulfide and titanium disulfide. The main purpose is for electronic and opto electronic applications and devices, as well as basic physics research.

Q: Can you tell us about the study you are doing using an AFM?

We use Park AFM [Atomic Force Microscope] to locate and characterize the graphene and other 2D materials on a substrate. After we make the graphene device, we suspend the graphene by removing the substrate beneath the nano ribbons. We use the AFM to see whether the graphene
nano-ribbons are successfully suspended. It turns out that the AFM is pretty handy in determining that. If we don’t use the AFM, we would have to use the SEM [Scanning Electron Microscope]. The SEM would cause contamination.

Q: What made AFM the best choice for your study?

I use AFM because it is accurate, and it is noninvasive [Park AFM non-contact mode]. With SEM, it is hard to avoid all deposits on those materials and devices that are mostly carbons which alter the electronic properties. If we use the AFM, it’s clean. You don’t introduce anything. Furthermore, AFM allows us to measure the thickness and the width of our materials accurately. We sometimes measure even the length which otherwise we’d have to use either the SEM or rely on the optical microscope image which is not as accurate as an AFM.

Q: How well is Park AFM meeting your research needs?

Good! I like [Park] XE-70 AFM because it’s very user friendly. It’s easy to operate, and the operation is quite intuitive. For instance, the approaching process with your system [Park AFM] is very straight forward and intuitive. A new student without any prior experience can learn to use it, and generate sample measurements within a week.

Q: What are the key features of Park AFM do you like the most?

We always use the non-contact mode so the tip will last longer. Also, because accurate measurement on the thickness or width dimension is very important, we always use the closed loop [feature on Park AFM] measurement.

Q: I understand that you are doing control nano-cutting with scanning probe tips?

Not yet. We plan to study several samples with different domains showing somewhat different local conductivity, insulating or less conductive than other result by a transport property measurement with whole device. We are expecting to have further understanding on the local electrical properties using scanning probe techniques because sometimes, the formation of domains could be a critical factor to determine the electronic properties of the devices. One of the basic ideas we are approaching is to use conductive AFM for studying local electronic property while measuring transport measurement at the same time.

About the Research Using Park AFM:

Room-temperature high on/off ratio in suspended graphene nanoribbon field-effect transistors
We have fabricated suspended few-layer (1–3 layers) graphene nanoribbon field-effect transistors from unzipped multi-wall carbon nanotubes. Electrical transport measurements show that current annealing effectively removes the impurities on the suspended graphene nanoribbons, uncovering the intrinsic ambipolar transfer characteristic of graphene. Further increasing the annealing current creates a narrow constriction in the ribbon, leading to the formation of a large bandgap and subsequent high on/off ratio (which can exceed 104) even at room temperature. Such fabricated devices are thermally and mechanically stable: repeated thermal cycling has little effect on their electrical properties. This work shows for the first time that ambipolar field-effect characteristics and high on/off ratios at room temperature can be achieved in relatively wide graphene nanoribbons (15–50 nm) by controlled current annealing.

Electrical transport properties of graphene nanoribbons produced from sonicating graphite in solution

A simple one-stage solution-based method was developed to produce graphene nanoribbons by sonicating graphite powder in organic solutions with polymer surfactant. The graphene nanoribbons were deposited on a silicon substrate, and characterized by Raman spectroscopy and atomic force microscopy. Single-layer and few-layer graphene nanoribbons with a width ranging from sub-10 nm to tens of nanometers and lengths ranging from hundreds of nanometers to one micron were routinely observed. The electrical transport properties of individual graphene nanoribbons were measured in both the back-gate and polymer–electrolyte top-gate configurations. The mobility of the graphene nanoribbons was found to be over an order of magnitude higher when measured in the latter than in the former configuration (without the polymer–electrolyte), which can be attributed to the screening of the charged impurities by the counter ions in the polymer–electrolyte. This finding suggests that the charge transport in the solution produced graphene nanoribbons is largely limited by charge impurity scattering.


Published Work Using Park AFM

1) Room-temperature high on/off ratio in suspended graphene nanoribbon field-effect transistors Ming-Wei Lin1,4, Cheng Ling1,4, Yiyang Zhang1,2, Hyeun Joong Yoon2, Mark Ming-Cheng, Cheng2, Luis A Agapito3, Nicholas Kioussis3, NoppiWidjaja1 and Zhixian Zhou1,5
1 Department of Physics and Astronomy, Wayne State University, Detroit, MI 48201, USA
2 Department of Electrical and Computer Engineering, Wayne State University, Detroit,
MI 48202, USA
Published 17 May 2011 Online at stacks.iop.org/Nano/22/265201
2) Electrical transport properties of graphene nanoribbons produced from sonicating graphite in
solution Cheng Ling1,3, Gabriel Setzler1,2,3, Ming-Wei Lin1,3, Kulwinder Singh Dhindsa1, Jin
Jin1, Hyeun Joong Yoon2, Seung Soo Kim1,2, Mark Ming-Cheng Cheng2, NoppiWidjaja1 and
Zhixian Zhou1,4
1 Department of Physics and Astronomy, Wayne State University, Detroit, MI 48201, USA
2 Department of Electrical and Computer Engineering, Wayne State University, Detroit,
MI 48202, USA
Published 14 July 2011 Online at stacks.iop.org/Nano/22/325201
3) Ming-Wei Lin1,#, Cheng Ling1,#, Luis A. Agapito2,#, Nicholas Kioussis2, Yiyang Zhang1,3,
Mark Ming-Cheng Cheng3,Wei L. Wang4, Efthimios Kaxiras4and Zhixian Zhou1,*
Approaching the Intrinsic Bandgap in Suspended High-Mobility Graphene Nanoribbons
(2011) Phys. Rev. B 84, 125411 (2011)

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