Qingzhou Liu

Red Phosphorus Nanodots on Reduced Graphene Oxide as a Flexible and Ultra-Fast Anode for Sodium-Ion Batteries

Sodium-ion batteries offer an attractive option for potential low cost and large scale energy storage due to the earth abundance of sodium. Red phosphorus is considered as a high capacity anode for sodium-ion batteries with a theoretical capacity of 2596 mAh/g. However, similar to silicon in lithium-ion batteries, several limitations, such as large volume expansion upon sodiation/desodiation and low electronic conductance, have severely limited the performance of red phosphorus anodes. In order to address the above challenges, we have developed a method to deposit red phosphorus nanodots densely and uniformly onto reduced graphene oxide sheets (P@RGO) to minimize the sodium ion diffusion length and the sodiation/desodiation stresses, and the RGO network also serves as electron pathway and creates free space to accommodate the volume variation of phosphorus particles. The resulted P@RGO flexible anode achieved 1165.4, 510.6, and 135.3 mAh/g specific charge capacity at 159.4, 31878.9, and 47818.3 mA/g charge/discharge current density in rate capability test, and a 914 mAh/g capacity after 300 deep cycles in cycling stability test at 1593.9 mA/g current density, which marks a significant performance improvement for red phosphorus anodes for sodium-ion chemistry and flexible power sources for wearable electronics.

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.

Black Phosphorus Field-Effect Transistors with Work Function Tunable Contacts

Black phosphorus (BP) has been recently rediscovered as an elemental two-dimensional (2D) material that shows promising results for next generation electronics and optoelectronics because of its intrinsically superior carrier mobility and small direct band gap. In various 2D field-effect transistors (FETs), the choice of metal contacts is vital to the device performance, and it is a major challenge to reach ultralow contact resistances for highly scaled 2D FETs. Here, we experimentally show the effect of a work function tunable metal contact on the device performance of BP FETs. Using palladium (Pd) as the contact material, we employed the reaction between Pd and H2 to form a Pd-H alloy that effectively increased the work function of Pd and reduced the Schottky barrier height (ΦB) in a BP FET. When the Pd-contacted BP FET was exposed to 5% hydrogen concentrated Ar, the contact resistance (Rc) improved between the Pd electrodes and BP from ∼7.10 to ∼1.05 Ω·mm, surpassing all previously reported contact resistances in the literature for BP FETs. Additionally, with exposure to 5% hydrogen, the transconductance of the Pd-contacted BP FET was doubled. The results shown in this study illustrate the significance of choosing the right contact material for high-performance BP FETs in order to realize the real prospect of BP in electronic applications.

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.

Aligned Carbon Nanotube Synaptic Transistors for Large-Scale Neuromorphic Computing

This paper presents aligned carbon nanotube (CNT) synaptic transistors for large-scale neuromorphic computing systems. The synaptic behavior of these devices is achieved via charge-trapping effects, commonly observed in carbon-based nanoelectronics. In this work, charge trapping in the high- k dielectric layer of top-gated CNT field-effect transistors (FETs) enables the gradual analog programmability of the CNT channel conductance with a large dynamic range ( i. e., large on/off ratio). Aligned CNT synaptic devices present significant improvements over conventional memristor technologies ( e. g., RRAM), which suffer from abrupt transitions in the conductance modulation and/or a small dynamic range. Here, we demonstrate exceptional uniformity of aligned CNT FET synaptic behavior, as well as significant robustness and nonvolatility via pulsed experiments, establishing their suitability for neural network implementations. Additionally, this technology is based on a wafer-level technique for constructing highly aligned arrays of CNTs with high semiconducting purity and is fully CMOS compatible, ensuring the practicality of large-scale CNT+CMOS neuromorphic systems. We also demonstrate fine-tunability of the aligned CNT synaptic behavior and discuss its application to adaptive online learning schemes and to homeostatic regulation of artificial neuron firing rates. We simulate the implementation of unsupervised learning for pattern recognition using a spike-timing-dependent-plasticity scheme, indicate system-level performance (as indicated by the recognition accuracy), and demonstrate improvements in the learning rate resulting from tuning the synaptic characteristics of aligned CNT devices.

Room-Temperature Pressure Synthesis of Layered Black Phosphorus–Graphene Composite for Sodium-Ion Battery Anodes

Sodium-ion batteries offer an attractive option for grid-level energy storage due to the high natural abundance of sodium and low material cost of sodium compounds. Phosphorus (P) is a promising anode material for sodium-ion batteries, with a theoretical capacity of 2596 mAh/g. The red phosphorus (RP) form has worse electronic conductivity and lower initial Coulombic efficiency than black phosphorus (BP), but high material cost and limited production capacity have slowed the development of BP anodes. To address these challenges, we have developed a simple and scalable method to synthesize layered BP/graphene composite (BP/rGO) by pressurization at room temperature. A carbon-black-free and binder-free BP/rGO anode prepared with this method achieved specific charge capacities of 1460.1, 1401.2, 1377.6, 1339.7, 1277.8, 1123.78, and 720.8 mAh/g in a rate capability test at charge and discharge current densities of 0.1, 0.5, 1, 5, 10, 20, and 40 A/g, respectively. In a cycling performance test, after 500 deep cycles, the capacity of BP/rGO anodes stabilized at 1250 and 640 mAh/g at 1 and 40 A/g, respectively, which marks a significant performance improvement for sodium-ion battery anodes.

Single-step flash-heat synthesis of red phosphorus/graphene flame-retardant composite as flexible anodes for sodium-ion batteries

Red phosphorus (RP) has attracted considerable attention as the anode for high-performance Na-ion batteries, owing to its low cost and high theoretical specific capacity of ∼ 2,600 mAh/g. In this study, a facile single-step flash-heat treatment was developed to achieve the reduction of graphene oxide (GO) and the simultaneous deposition of RP onto the reduced graphene oxide (rGO) sheets. The resulting RP/rGO composite was shown to be a promising candidate for overcoming the issues associated with the poor electronic conductivity and large volume variation of RP during cycling. The RP/rGO flexible film anode delivered an average capacity of 1,625 mAh/g during 200 cycles at a charge/discharge current density of 1 A/g. Average charge capacities of 1,786, 1,597, 1,324, and 679 mAh/g at 1, 2, 4, and 6 A/g current densities were obtained in the rate capability tests. Moreover, owing to the RP component, the RP/rGO film presented superior flame retardancy compared to an rGO film. This work thus introduces a highly accessible synthesis method to prepare flexible and safe RP anodes with superior electrochemical performance toward Na-ion storage.

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.