Browsing by Author "Ding, Lei"

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Open Access
Orthogonal orientation control of carbon nanotube growth.
(J Am Chem Soc, 2010-01-13) Zhou, Weiwei; Ding, Lei; Yang, Sungwoo; Liu, Jie
Carbon nanotubes (CNTs) have attracted attention for their remarkable electrical properties and have being explored as one of the best building blocks in nano-electronics. A key challenge to realize such potential is the control of the nanotube growth directions. Even though both vertical growth and controlled horizontal growth of carbon nanotubes have been realized before, the growth of complex nanotube structures with both vertical and horizontal orientation control on the same substrate has never been achieved. Here, we report a method to grow three-dimensional (3D) complex nanotube structures made of vertical nanotube forests and horizontal nanotube arrays on a single substrate and from the same catalyst pattern by an orthogonally directed nanotube growth method using chemical vapor deposition (CVD). More importantly, such a capability represents a major advance in controlled growth of carbon nanotubes. It enables researchers to control the growth directions of nanotubes by simply changing the reaction conditions. The high degree of control represented in these experiments will surely make the fabrication of complex nanotube devices a possibility.
Open Access
Piezopotential gated nanowire--nanotube hybrid field-effect transistor.
(Nano Lett, 2010-08-11) Liu, Weihua; Lee, Minbaek; Ding, Lei; Liu, Jie; Wang, Zhong Lin
We report the first piezoelectric potential gated hybrid field-effect transistors based on nanotubes and nanowires. The device consists of single-walled carbon nanotubes (SWNTs) on the bottom and crossed ZnO piezoelectric fine wire (PFW) on the top with an insulating layer between. Here, SWNTs serve as a carrier transport channel, and a single-crystal ZnO PFW acts as the power-free, contact-free gate or even an energy-harvesting component later on. The piezopotential created by an external force in the ZnO PFW is demonstrated to control the charge transport in the SWNT channel located underneath. The magnitude of the piezopotential in the PFW at a tensile strain of 0.05% is measured to be 0.4-0.6 V. The device is a unique coupling between the piezoelectric property of the ZnO PFW and the semiconductor performance of the SWNT with a full utilization of its mobility. The newly demonstrated device has potential applications as a strain sensor, force/pressure monitor, security trigger, and analog-signal touch screen.
Open Access
The Use of a Novel Radiation Detector on Quantifying PET/Nuclear Medicine Occupational and Non-occupational Doses and Calibration of MOSFET Radiation Detectors against Effective Energy
(2013) Ding, Lei
Project 1: Dose reduction for PET technologists by the automatic dose draw/injection system
* Purpose: To evaluate the dose reduction by the installed automatic dose draw/injection machine.
* Materials & Methods: Six RadEye detectors were given to six PET technologists. A RadEye detector recorded data every 25 seconds throughout the day. Technologists logged their activities as follows: dose draw/injection, patient positioning, patient transport, patient care and non-specific. One technologist performed dose drawing/injection manually while others used the Trasis system. The Trasis machine was monitored with a Radeye detector during the period as well.
* Results: The average dose reduction brought by Trasis is 75% for dose draw and 70% for dose injection. Qualitatively, instead of dose draw/injection, patient positioning has become the most significant contributing factor to overall PET technologist dose. In addition, the current average daily dose for a PET technologist is about 0.03 mSv, which on average is 36% less than before [12]. PET technologists typically received a dose of 0.007 mSv from dose draw/injection, 0.005 mSv from patient transport, 0.013 mSv from patient positioning, 0.001 mSv from patient care, and 0.003 mSv from non-specific per working day. This would result in an annual dose of 8 mSv which is approximately 16% of occupational dose limit (50 mSv).
* Conclusions: The installation of automatic dose draw/injection machine has clear benefits to the PET technologists. The radiation doses for PET technologists are well within the annual limit of doses to occupational radiation workers.
Project 2: Validation of ceiling shielding in CT/PET room with RADEYE
* Purpose: To measure and the magnitude of scattered radiation levels in CT/PET suite and to evaluate the effectiveness of shielding of the ceiling.
* Materials & Methods: Six RadEye detectors were placed in the CT/PET room, four in the ceiling, and two at one meter above floor. A RadEye detector recorded data every 25 seconds throughout the day. The detector was turned on at the beginning of the day (6 am) and the doses were transferred to a laptop for analysis at the end of the day (5 pm). The dose to a non-radiation worker above the CT/PET room was estimated based on the ceiling data. The magnitude of transmitted CT radiation in the room above was measured separately with RadEye.
* Results: The CT dose contributed about 80% of the total dose while the PET contributed 20% within the scanning room. No dose contribution was measured above the floor from CT scanning. The combined dose from both PET and CT scan in a room above at 2.2 meter was 2.4x10-6 mSv per week, assuming an occupancy factor of 1.
* Conclusions: This study quantified the CT and PET doses contributions separately in the clinical CT/PET room. An analytical model was developed to calculate the non-occupational personnel dose above the CT/PET room and the calculated results were confirmed by physical measurements. The actual physical dose was much smaller than the NCRP design goals of 0.02 mSv/wk.
Project 3: Evaluating MOSFET dependency on effective energy over diagnostic energy range
* Purpose: To characterize MOSFET calibration factors (CF) as a function of effective energy over diagnostic energy range.
* Materials & Methods: Five new MOSFETs were used in the study. The calibration factors were measured in two ways: 1) fixed kVp, fixed SSD, fixed FOV, and varying filtration; and 2) fixed filtration, fixed SSD, fixed FOV, and varying kVp. Effective energy was computed as a function of kVp and filtration by SpekCalc.
* Results: CF was independent with HVL in the range of HVL = 5 to HVL = 9mm Al at a fixed 120 kVp. CF depended linearly with kilo-voltage (kVp) from 80 to 140 kVp at a fixed filtration. In addition, a strong non-linear correlation of average CF versus effective energy was generated for effective energies in the diagnostic range (Goodness of fit of 0.98).
* Conclusions: A correlation of second degree polynomial was seen between calibration factor and effective energy over diagnostic range. Hence, we created a calibration curve so that under a given fixed kVp or filtration, the calibration factor is automatically generated. A high correlation between CF versus effective energy was found over the diagnostic energy from 45 keV to 65 keV. This suggests that we could estimate the calibration factor with in-house generated MOSFET aging data, which would have a direct impacted CF linearly.