Fanqi Wu

Screen printing as a scalable and low-cost approach for rigid and flexible thin-film transistors using separated carbon nanotubes.

Semiconducting single-wall carbon nanotubes are very promising materials in printed electronics due to their excellent mechanical and electrical property, outstanding printability, and great potential for flexible electronics. Nonetheless, developing scalable and low-cost approaches for manufacturing fully printed high-performance single-wall carbon nanotube thin-film transistors remains a major challenge. Here we report that screen printing, which is a simple, scalable, and cost-effective technique, can be used to produce both rigid and flexible thin-film transistors using separated single-wall carbon nanotubes. Our fully printed top-gated nanotube thin-film transistors on rigid and flexible substrates exhibit decent performance, with mobility up to 7.67 cm2 V(-1) s(-1), on/off ratio of 10(4)∼10(5), minimal hysteresis, and low operation voltage (<10 V). In addition, outstanding mechanical flexibility of printed nanotube thin-film transistors (bent with radius of curvature down to 3 mm) and driving capability for organic light-emitting diode have been demonstrated. Given the high performance of the fully screen-printed single-wall carbon nanotube thin-film transistors, we believe screen printing stands as a low-cost, scalable, and reliable approach to manufacture high-performance nanotube thin-film transistors for application in display electronics. Moreover, this technique may be used to fabricate thin-film transistors based on other materials for large-area flexible macroelectronics, and low-cost display electronics.

Chirality-Controlled Synthesis and Applications of Single-Wall Carbon Nanotubes

Preparation of chirality-defined single-wall carbon nanotubes (SWCNTs) is the top challenge in the nanotube field. In recent years, great progress has been made toward preparing single-chirality SWCNTs through both direct controlled synthesis and postsynthesis separation approaches. Accordingly, the uses of single-chirality-dominated SWCNTs for various applications have emerged as a new front in nanotube research. In this Review, we review recent progress made in the chirality-controlled synthesis of SWCNTs, including metal-catalyst-free SWCNT cloning by vapor-phase epitaxy elongation of purified single-chirality nanotube seeds, chirality-specific growth of SWCNTs on bimetallic solid alloy catalysts, chirality-controlled synthesis of SWCNTs using bottom-up synthetic strategy from carbonaceous molecular end-cap precursors, etc. Recent major progresses in postsynthesis separation of single-chirality SWCNT species, as well as methods for chirality characterization of SWCNTs, are also highlighted. Moreover, we discuss some examples where single-chirality SWCNTs have shown clear advantages over SWCNTs with broad chirality distributions. We hope this review could inspire more research on the chirality-controlled preparation of SWCNTs and equally important inspire the use of single-chirality SWCNT samples for more fundamental studies and practical applications.

Threshold voltage tuning and printed complementary transistors and inverters based on thin films of carbon nanotubes and indium zinc oxide

Carbon nanotubes (CNTs) have emerged as an important material for printed macroelectronics. However, achieving printed complementary macroelectronics solely based on CNTs is difficult because it is still challenging to make reliable n-type CNT transistors. In this study, we report threshold voltage (Vth) tuning and printing of complementary transistors and inverters composed of thin films of CNTs and indium zinc oxide (IZO) as p-type and n-type transistors, respectively. We have optimized the Vth of p-type transistors by comparing Ti/Au and Ti/Pd as source/drain electrodes, and observed that CNT transistors with Ti/Au electrodes exhibited enhancement mode operation (Vth < 0). In addition, the optimized In:Zn ratio offers good n-type transistors with high on-state current (Ion) and enhancement mode operation (Vth > 0). For example, an In:Zn ratio of 2:1 yielded an enhancement mode n-type transistor with Vth ∼ 1 V and Ion of 5.2 μA. Furthermore, by printing a CNT thin film and an IZO thin film on the same substrate, we have fabricated a complementary inverter with an output swing of 99.6% of the supply voltage and a voltage gain of 16.9. This work shows the promise of the hybrid integration of p-type CNT and n-type IZO for complementary transistors and circuits.

Top-Contact Self-Aligned Printing for High-Performance Carbon Nanotube Thin-Film Transistors with Sub-Micron Channel Length

Semiconducting single-wall carbon nanotubes are ideal semiconductors for printed thin-film transistors due to their excellent electrical performance and intrinsic printability with solution-based deposition. However, limited by resolution and registration accuracy of current printing techniques, previously reported fully printed nanotube transistors had rather long channel lengths (>20 μm) and consequently low current-drive capabilities (<0.2 μA/μm). Here we report fully inkjet printed nanotube transistors with dramatically enhanced on-state current density of ∼4.5 μA/μm by downscaling the devices to a sub-micron channel length with top-contact self-aligned printing and employing high-capacitance ion gel as the gate dielectric. Also, the printed transistors exhibited a high on/off ratio of ∼105, low-voltage operation, and good mobility of ∼15.03 cm2 V-1s-1. These advantageous features of our printed transistors are very promising for future high-definition printed displays and sensing systems, low-power consumer electronics, and large-scale integration of printed electronics.

Review of Electronics Based on Single-Walled Carbon Nanotubes

Single-walled carbon nanotubes (SWNTs) are extremely promising materials for building next-generation electronics due to their unique physical and electronic properties. In this article, we will review the research efforts and achievements of SWNTs in three electronic fields, namely analog radio-frequency electronics, digital electronics, and macroelectronics. In each SWNT-based electronic field, we will present the major challenges, the evolutions of the methods to overcome these challenges, and the state-of-the-art of the achievements. At last, we will discuss future directions which could lead to the broad applications of SWNTs. We hope this review could inspire more research on SWNT-based electronics, and accelerate the applications of SWNTs.

Highly Sensitive and Wearable In2O3 Nanoribbon Transistor Biosensors with Integrated On-Chip Gate for Glucose Monitoring in Body Fluids

Nanoribbon- and nanowire-based field-effect transistor (FET) biosensors have stimulated a lot of interest. However, most FET biosensors were achieved by using bulky Ag/AgCl electrodes or metal wire gates, which have prevented the biosensors from becoming truly wearable. Here, we demonstrate highly sensitive and conformal In2O3 nanoribbon FET biosensors with a fully integrated on-chip gold side gate, which have been laminated onto various surfaces, such as artificial arms and watches, and have enabled glucose detection in various body fluids, such as sweat and saliva. The shadow-mask-fabricated devices show good electrical performance with gate voltage applied using a gold side gate electrode and through an aqueous electrolyte. The resulting transistors show mobilities of ∼22 cm2 V-1 s-1 in 0.1× phosphate-buffered saline, a high on-off ratio (105), and good mechanical robustness. With the electrodes functionalized with glucose oxidase, chitosan, and single-walled carbon nanotubes, the glucose sensors show a very wide detection range spanning at least 5 orders of magnitude and a detection limit down to 10 nM. Therefore, our high-performance In2O3 nanoribbon sensing platform has great potential to work as indispensable components for wearable healthcare electronics.

Quasi-two-dimensional β-Ga2O3 field effect transistors with large drain current density and low contact resistance via controlled formation of interfacial oxygen vacancies

AbstractQuasi-two-dimensional (2D) β-Ga2O3 is a rediscovered metal-oxide semiconductor with an ultra-wide bandgap of 4.6–4.9 eV. It has been reported to be a promising material for next-generation power and radio frequency electronics. Field effect transistors (FETs) that can switch at high voltage are key components in power and radio frequency devices, and reliable Ohmic contacts are essential for high FET performance. However, obtaining low contact resistance on β-Ga2O3 FETs is difficult since reactions between β-Ga2O3 and metal contacts are not fully understood. Herein, we experimentally demonstrate the importance of reactions at the metal/β-Ga2O3 interface and the corresponding effects of these reactions on FET performance. When Ti is employed as the metal contact, annealing of β-Ga2O3 FETs in argon can effectively transform Schottky contacts into Ohmic contacts and permit a large drain current density of ~ 3.1 mA/μm. The contact resistance (Rcontact) between the Ti electrodes and β-Ga2O3 decreased from ~ 430 to ~ 0.387 Ω·mm after annealing. X-ray photoelectron spectroscopy (XPS) confirmed the formation of oxygen vacancies at the Ti/β-Ga2O3 interface after annealing, which is believed to cause the improved FET performance. The results of this study pave the way for greater application of β-Ga2O3 in electronics.