Guanhong Li

The Dependence of Graphene Raman D-band on Carrier Density

Raman spectroscopy has been an integral part of graphene research and can provide information about graphene structure, electronic characteristics, and electron-phonon interactions. In this study, the characteristics of the graphene Raman D-band, which vary with carrier density, are studied in detail, including the frequency, full width half-maximum, and intensity. We find the Raman D-band frequency increases for hole doping and decreases for electron doping. The Raman D-band intensity increases when the Fermi level approaches half of the excitation energy and is higher in the case of electron doping than that of hole doping. These variations can be explained by electron-phonon interaction theory and quantum interference between different Raman pathways in graphene. The intensity ratio of Raman D- and G-band, which is important for defects characterization in graphene, shows a strong dependence on carrier density.

Three-Dimensional Flexible Complementary Metal–Oxide–Semiconductor Logic Circuits Based On Two-Layer Stacks of Single-Walled Carbon Nanotube Networks

We have proposed and fabricated stable and repeatable, flexible, single-walled carbon nanotube (SWCNT) thin film transistor (TFT) complementary metal-oxide-semiconductor (CMOS) integrated circuits based on a three-dimensional (3D) structure. Two layers of SWCNT-TFT devices were stacked, where one layer served as n-type devices and the other one served as p-type devices. On the basis of this method, it is able to save at least half of the area required to construct an inverter and make large-scale and high-density integrated CMOS circuits easier to design and manufacture. The 3D flexible CMOS inverter gain can be as high as 40, and the total noise margin is more than 95%. Moreover, the input and output voltage of the inverter are exactly matched for cascading. 3D flexible CMOS NOR, NAND logic gates, and 15-stage ring oscillators were fabricated on PI substrates with high performance as well. Stable electrical properties of these circuits can be obtained with bending radii as small as 3.16 mm, which shows that such a 3D structure is a reliable architecture and suitable for carbon nanotube electrical applications in complex flexible and wearable electronic devices.

Trap-state-dominated suppression of electron conduction in carbon nanotube thin-film transistors.

The often observed p-type conduction of single carbon nanotube field-effect transistors is usually attributed to the Schottky barriers at the metal contacts induced by the work function differences or by the doping effect of the oxygen adsorption when carbon nanotubes are exposed to air, which cause the asymmetry between electron and hole injections. However, for carbon nanotube thin-film transistors, our contrast experiments between oxygen doping and electrostatic doping demonstrate that the doping-generated transport barriers do not introduce any observable suppression of electron conduction, which is further evidenced by the perfect linear behavior of transfer characteristics with the channel length scaling. On the basis of the above observation, we conclude that the environmental adsorbates work by more than simply shifting the Fermi level of the CNTs; more importantly, these adsorbates cause a poor gate modulation efficiency of electron conduction due to the relatively large trap state density near the conduction band edge of the carbon nanotubes, for which we further propose quantitatively that the adsorbed oxygen-water redox couple is responsible.

Study of Carbon Nanotubes as Etching Masks and Related Applications in the Surface Modification of GaAs-based Light-Emitting Diodes

The surface modification of LEDs based on GaAs is realized by super-aligned multiwalled carbon nanotube (SACNT) networks as etching masks. The surface morphology of SACNT networks is transferred to the GaAs. It is found that the light output power of LEDs based on GaAs with a nanostructured surface morphology is greatly enhanced with the electrical power unchanged.

Modeling and optimization of ambipolar graphene transistors in the diffusive limit

We derived an analytical expression based on the Pao-Sah theory to characterize the electric conduction of ambipolar graphene transistors. We included and solved exactly the contact resistance, thermal excitation of carrier density, quantum capacitance, and the velocity saturation effect. Our model agreed with the experimental results for ion-gel gated graphene transistors. The microscopic conduction behavior was calculated and proved to be helpful for understanding the weak current saturation observed because of the “kink effect.” To achieve a high voltage gain for radio-frequency or analog circuit applications, the carrier velocity should be facilitated to reach saturation before the formation of the minimal carrier point inside the channel, which can be realized by decreasing the channel length and the series contact resistance. Given a finite channel length and the series contact resistance, the optimized gate capacitance can be solved analytically. Considering state-of-the-art device parameters, we f...

Demonstration of nonvolatile multilevel memory in ambipolar carbon nanotube thin-film transistors

Multilevel memories have attracted significant interest because of their larger memory density per unit cell. Here, we investigated multilevel operation with ambipolar carbon nanotube thin-film transistors. Three distinct conduction states and a direct change between any of them were demonstrated by selecting appropriate values for the magnitude and duration of each program/erase voltage pulse. A low operation voltage of 5 V and a short duration of 1 s were obtained by utilizing a bilayer Al2O3-epoxy dielectric to enhance the gate modulation efficiency. A tradeoff exists between low-voltage operation and fast switching for a given device.