| 1. |
- Liu, Zhiying, et al.
(författare)
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Hysteresis-free thin-film transistors achieved by novel solution-processing of nanotubes/polymer composites
- 2012
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Ingår i: Materials Research Society Spring Meeting 2012, San Francisco, April 9-13, 2012..
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Konferensbidrag (refereegranskat)abstract
- Thin-film transistors (TFTs) based on single-walled carbon nanotubes (SWCNTs) have gained enormous attention in the community of flexible/stretchable electronics. At present, such TFTs often suffer from severe problems including giant hysteresis in their transfer characteristics. With SiO2 as the gate dielectric, extensive investigations have led to generally accepted understanding of the hysteresis as being caused by charge transfer between the SWCNTs and their surroundings including both water molecules bound on the SiO2 surface (Si≡OH) and the water/oxygen molecules in the ambient atmosphere. In order to combat the hysteresis issue, significant efforts have been made by annealing the TFTs in vacuum and separating SWCNTs from SiO2 by deposition of a self-assembled monolayer (SAM) on the SiO2 or passivating the SWCNTs with an organic or inorganic dielectric film. These methods, however, require either processing in inert environment or developing elaborated processes. In the present work, we demonstrate hysteresis-free TFTs based on SWCNT/polymer composite without any complex treatment. The composite consists of SWCNTs and poly-9,9_dioctyl-fluorene-co-bithiophene (F8T2). With the aid of polymer F8T2, SWCNTs can be efficiently dissolved in commonly used solvents thereby forming a uniform composite solution. By soaking a chip with predefined TFT structures on an oxidized Si substrate in the composite solution, direct assembly of the composite on the SiO2 occurs, leading to the formation of a composite thin film in the channel region of the TFTs. Although fabricated using a very simple process, our TFTs exhibit hysteresis-free operation under ambient conditions. It is plausible to suggest that SWCNTs are embedded in the F8T2 matrix with the latter providing an effective shield for the former against the trap sites on the SiO2 and the H2O/O2 molecules in the atmosphere. In comparison to the other reported means aiming at hysteresis reduction, the present method is simple, robust, solution processable, effective, and operable under ambient conditions. In addition, we have found F8T2 to preferentially disperse semiconducting SWCNTs rendering a selective removal of the metallic species in the solution. This selectivity is of paramount importance as it results in high-performance TFTs with both high on-state current (0.1 µA/µm @ channel length = 50 µm) and large on/off current ratio (103-105). The TFTs have also shown significantly improved uniformity and dimensional scalability with a mobility value of 10-20 cm2V-1s-1, which have allowed us to investigate the TFTs using the resultant logic circuits.
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| 2. |
- Qu, Minni, et al.
(författare)
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Charge-Injection-Induced Time Decay in Carbon Nanotube Network-Based FETs
- 2010
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Ingår i: IEEE Electron Device Letters. - 0741-3106 .- 1558-0563. ; 31:10, s. 1098-1100
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Tidskriftsartikel (refereegranskat)abstract
- A voltage-pulse method is utilized to investigate the charge-injection-induced time decay of the source-drain current of field-effect transistors with randomly networked single-walled carbon nanotubes (CNTs) as the conduction channel. The relaxation of trapped carriers in the CNT networks can be accounted for by assuming two exponential decays occurring simultaneously. The slow decay is characterized by a time constant comparable to literature data obtained for a carrier recombination in the semiconducting CNTs. The faster decay with a time constant that has a smaller order of magnitude is attributed to the annihilation of trapped carriers in metallic CNTs or at metal-CNT contacts. Both time constants are gate-bias dependent.
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| 3. |
- Li, Zhiyuan, 1974, et al.
(författare)
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Cost–benefit analysis of a trans-arctic alternative route to the Suez canal: a method based on high-fidelity ship performance, weather, and ice forecast models
- 2023
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Ingår i: Journal of Marine Science and Engineering. - : MDPI AG. - 2077-1312. ; 11:4
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Tidskriftsartikel (refereegranskat)abstract
- Climate change in recent years has produced viable shipping routes in the Arctic. However, critical uncertainties related to maritime operations in the Arctic make it difficult to predict ship speeds in ice and, thus, the voyage time and fuel costs. Cost–benefit analysis of alternative Arctic routes based on accurate environmental condition modeling is required. In this context, this paper presents a holistic approach that considers the major voyage-related costs of a trans-Arctic route as an alternative to the conventional routes via the Suez Canal Route (SCR) for existing merchant ships. This tool is based on high-fidelity models of ship performance, metocean forecasting, and a voyage optimization algorithm. Case studies are performed based on a general cargo vessel in operation to quantify realistic expenses inclusive of all the major operational, fuel, and voyage costs of the specific voyages. A comparison is made between the total costs of the trans-Arctic route and SCR for different seasons, which proves the economic feasibility of the trans-Arctic route. Overall, this work can provide valuable insights to help policymakers as well as shipbuilders, owners, and operators to assess the potential cost-effectiveness and sustainability of future Arctic shipping, thereby better developing future strategies.
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| 4. |
- Chen, Hui, et al.
(författare)
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Hyperbolic CORDIC-Based Architecture for Computing Logarithm and Its Implementation
- 2020
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Ingår i: IEEE Transactions on Circuits and Systems - II - Express Briefs. - : Institute of Electrical and Electronics Engineers (IEEE). - 1549-7747 .- 1558-3791. ; 67:11, s. 2652-2656
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Tidskriftsartikel (refereegranskat)abstract
- We present a CORDIC (Coordinate Rotation Digital Computer)-based method to compute the logarithm function with base 2 and validate this method by software simulation and hardware implementation. Technically, we overcome the limitation of traditional hyperbolic CORDIC and transform it based on the idea of generalized hyperbolic CORDIC so that it can be used to compute $log_{2}x\;(x\;\epsilon \;[1,2))$ . The proposed method requires only simple shift-and-add operations and has a great tradeoff between precision (or speed) and area. In MATLAB, we provide different precisions corresponding to the iterations of the transformed CORDIC for user needs. Using a pipelined structure and setting the number of iterations to be 16 (the average relative error is $2.09\times 10<^>{-6}$ ), we implement an example hardware circuit. Synthesized under the SMIC 65nm CMOS technology, the circuit has an area of 24100 $\mu m<^>{2}$ and computation time of 11.1 ns, which can save 31.04x0025; area and improve 6.92x0025; computation speed averagely compared with existing methods.
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| 5. |
- Li, Hui, et al.
(författare)
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Accelerating Gas Adsorption on 3D Percolating Carbon Nanotubes
- 2016
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Ingår i: Scientific Reports. - : Springer Science and Business Media LLC. - 2045-2322. ; 6
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Tidskriftsartikel (refereegranskat)abstract
- In the field of electronic gas sensing, low-dimensional semiconductors such as single-walled carbon nanotubes (SWCNTs) can offer high detection sensitivity owing to their unprecedentedly large surface-to-volume ratio. The sensitivity and responsivity can further improve by increasing their areal density. Here, an accelerated gas adsorption is demonstrated by exploiting volumetric effects via dispersion of SWCNTs into a percolating three-dimensional (3D) network in a semiconducting polymer. The resultant semiconducting composite film is evaluated as a sensing membrane in field effect transistor (FET) sensors. In order to attain reproducible characteristics of the FET sensors, a pulsed-gate-bias measurement technique is adopted to eliminate current hysteresis and drift of sensing baseline. The rate of gas adsorption follows the Langmuir-type isotherm as a function of gas concentration and scales with film thickness. This rate is up to 5 times higher in the composite than only with an SWCNT network in the transistor channel, which in turn results in a 7-fold shorter time constant of adsorption with the composite. The description of gas adsorption developed in the present work is generic for all semiconductors and the demonstrated composite with 3D percolating SWCNTs dispersed in functional polymer represents a promising new type of material for advanced gas sensors.
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| 6. |
- Liu, Zhiying, et al.
(författare)
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SMALL-Hysteresis Thin-Film Transistors Achieved by Facile Dip-Coating of Nanotube/Polymer Composite
- 2012
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Ingår i: Advanced Materials. - : Wiley. - 0935-9648 .- 1521-4095. ; 24:27, s. 3633-3638
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Tidskriftsartikel (refereegranskat)abstract
- Small-hysteresis, high-performance thin-film transistors (TFTs) are readily realized simply by dip-coating of a solution-processable composite. The composite consists of single-walled carbon nanotubes (SWCNTs) embedded in semiconducting polymer used as the channel material. The resultant TFTs simultaneously exhibit large on/off current ratio, high on-current level, high mobility in the range 10−20 cm2V−1s−1, and good uniformity and scalability.
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| 7. |
- Zhang, Youwei, et al.
(författare)
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On Valence-Band Splitting in Layered MoS2
- 2015
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Ingår i: ACS Nano. - : American Chemical Society (ACS). - 1936-0851 .- 1936-086X. ; 9:8, s. 8514-8519
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Tidskriftsartikel (refereegranskat)abstract
- As a representative two-dimensional semiconducting transition-metal dichalcogenide (TMD), the electronic structure in layered MoS2 is a collective result of quantum confinement, interlayer interaction, and crystal symmetry. A prominent energy splitting in the valence band gives rise to many intriguing electronic, optical, and magnetic phenomena. Despite numerous studies, an experimental determination of valence-band splitting in few-layer MoS2 is still lacking. Here, we show how the valence-band maximum (VBM) splits for one to five layers of MoS2. Interlayer coupling is found to contribute significantly to phonon energy but weakly to VBM splitting in bilayers, due to a small interlayer hopping energy for holes. Hence, spin-orbit coupling is still predominant in the splitting. A temperature-independent VBM splitting, known for single-layer MoS2, is, thus, observed for bilayers. However, a Bose-Einstein type of temperature dependence of VBM splitting prevails in three to five layers of MoS2. In such few-layer MoS2, interlayer coupling is enhanced with a reduced interlayer distance, but thermal expansion upon temperature increase tends to decouple adjacent layers and therefore decreases the splitting energy. Our findings that shed light on the distinctive behaviors about VBM splitting in layered MoS2 may apply to other hexagonal TMDs as well. They will also be helpful in extending our understanding of the TMD electronic structure for potential applications in electronics and optoelectronics.
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| 8. |
- Zhang, Youwei, et al.
(författare)
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Photothermoelectric and photovoltaic effects both present in MoS2
- 2015
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Ingår i: Scientific Reports. - : Springer Science and Business Media LLC. - 2045-2322. ; 5, s. 7938-
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Tidskriftsartikel (refereegranskat)abstract
- As a finite-energy-bandgap alternative to graphene, semiconducting molybdenum disulfide (MoS2) has recently attracted extensive interest for energy and sensor applications. In particular for broad-spectral photodetectors, multilayer MoS2 is more appealing than its monolayer counterpart. However, little is understood regarding the physics underlying the photoresponse of multilayer MoS2. Here, we employ scanning photocurrent microscopy to identify the nature of photocurrent generated in multilayer MoS2 transistors. The generation and transport of photocurrent in multilayer MoS2 are found to differ from those in other low-dimensional materials that only contribute with either photovoltaic effect (PVE) or photothermoelectric effect (PTE). In multilayer MoS2, the PVE at the MoS2-metal interface dominates in the accumulation regime whereas the hot-carrier-assisted PTE prevails in the depletion regime. Besides, the anomalously large Seebeck coefficient observed in multilayer MoS2, which has also been reported by others, is caused by hot photo-excited carriers that are not in thermal equilibrium with the MoS2 lattice.
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| 9. |
- Zhang, Youwei, et al.
(författare)
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Thickness Considerations of Two-Dimensional Layered Semiconductors for Transistor Applications
- 2016
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Ingår i: Scientific Reports. - : Springer Science and Business Media LLC. - 2045-2322. ; 6
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Tidskriftsartikel (refereegranskat)abstract
- Layered two-dimensional semiconductors have attracted tremendous attention owing to their demonstrated excellent transistor switching characteristics with a large ratio of on-state to off-state current, I-on/I-off. However, the depletion-mode nature of the transistors sets a limit on the thickness of the layered semiconductor films primarily determined by a given I-on/I-off as an acceptable specification. Identifying the optimum thickness range is of significance for material synthesis and device fabrication. Here, we systematically investigate the thickness-dependent switching behavior of transistors with a wide thickness range of multilayer-MoS2 films. A difference in I-on/I-off by several orders of magnitude is observed when the film thickness, t, approaches a critical depletion width. The decrease in I-on/I-off is exponential for t between 20 nm and 100 nm, by a factor of 10 for each additional 10 nm. For t larger than 100 nm, I-on/I-off approaches unity. Simulation using technical computer-aided tools established for silicon technology faithfully reproduces the experimentally determined scaling behavior of I-on/I-off with t. This excellent agreement confirms that multilayer-MoS2 films can be approximated as a homogeneous semiconductor with high surface conductivity that tends to deteriorate I-on/I-off. Our findings are helpful in guiding material synthesis and designing advanced field-effect transistors based on the layered semiconductors.
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| 10. |
- Chen, Hui, et al.
(författare)
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A CORDIC-Based Architecture with Adjustable Precision and Flexible Scalability to Implement Sigmoid and Tanh Functions
- 2020
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Ingår i: IEEE International Symposium on Circuits and Systems, ISCAS 2020. - : IEEE.
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Konferensbidrag (refereegranskat)abstract
- In the artificial neural networks, tanh (hyperbolic tangent) and sigmoid functions are widely used as activation functions. Past methods to compute them may have shortcomings such as low precision or inflexible architecture that is difficult to expand, so we propose a CORDIC-based architecture to implement sigmoid and tanh functions, which has adjustable precision and flexible scalability. It just needs shift-add-or-subtract operations to compute high-accuracy results and is easy to expand the input range through scaling the negative iterations of CORDIC without changing the original architecture. We adopt the control variable method to explore the accuracy distribution through software simulation. A specific case (ARCH:(1, 15, 18), RMSE: 10(-6)) is designed and synthesized under the TSMC 40nm CMOS technology, the report shows that it has the area of 36512.78 mu m(2) and power of 12.35mW at the frequency of 1GHz. The maximum work frequency can reach 1.5GHz, which is better than the state-of-the-art methods.
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