Fully solution-processed InSnO/HfGdOx thin-film transistor for light-stimulated artificial synapse

In recent years, the research interest in brain-inspired light-stimulated artificial synaptic electronic devices has greatly increased, due to their great potential in constructing low-power, high-efficiency, and high-speed neuromorphic computing systems. However, in the field of electronic synaptic device simulation, the development of three-terminal synaptic transistors with low manufacturing cost and excellent memory function still faces huge challenges. Here, a fully solution-processed InSnO/HfGdOx thin film transistor (TFT) is fabricated by a simple and convenient solution process to verify the feasibility of light-stimulated artificial synapses. This experiment investigated the electrical and synaptic properties of the device under light stimulation conditions. The device successfully achieved some important synaptic properties, such as paired-pulse facilitation (PPF), excitatory postsynaptic current (EPSC) and the transition from short-term memory (STM) to long-term memory (LTM). In addition, the device also exhibits brain-like memory and learning behaviors under different colors of light stimulation. This work provides an important strategy for the realization of light-stimulated artificial synapses and may have good applications in the field of artificial neuromorphic computing by light signals in the future.

Printed Zinc Tin Oxide Diodes: From Combustion Synthesis to Large-Scale Manufacturing

Printed metal oxide devices have been widely desired in flexible electronic applications to allow direct integration on foils and to reduce electronic waste and associated costs. Especially, semiconductor devices made from non-critical raw materials, such as Zn, Sn (and not, for example, In), have gained much interest. Despite considerable progress in the field, the upscale requirements from lab to fab scale to produce these materials and devices remain a challenge. In this work, we report the importance of solution combustion synthesis (SCS) when compared with sol-gel in the production of zinc tin oxide (ZTO) thin films using a solvent (1-methoxypropanol) that has lower environmental impact than the widely used and toxic 2-methoxyethanol. To assure the compatibility with low-cost flexible substrates in high-throughput printing techniques, a low annealing temperature of 140 ºC was achieved for these thin films by combining SCS and infrared (IR) annealing in a short processing time. These conditions allowed the transition from spin-coating (lab scale) to flexographic printing (fab scale) at a printing speed of 10 m/min in a roll-to-roll (R2R) pilot line. The ZTO (1:1 Zn:Sn-ratio) diodes show a rectification ratio of 103, a low operation voltage (≤ 3 V), promising reproducibility and low variability. The results provide the basis for further optimization (device size, encapsulation) to meet the requirements of diodes in flexible electronics applications such as passive-matrix addressing, energy harvesting and rectification.

A Stable 4-Bit ALU Design for Printed Devices

Printed devices fabricated using roll-to-roll (R2R) printing technology have been used in low-cost Internet of Things (IoTs), smart packaging and bio-chips. As the area of applications of printed devices broadens, arithmetic units in digital design need to be implemented. In this paper, we propose a stable 4-bit arithmetic logic unit (ALU) design using a minimum number of transistors that can overcome the limitations of printed devices. We propose the use of a 2:1 transmission gate (TG) multiplexer (MUX) structure and hybrid 16T full-adder to construct the ALU. New design methods are applied to reduce the number of inverter stages added to overcome the voltage degradation. Using this approach reduces the total number of transistors used in the design from 276 to 153, compared to the conventional design, with significant improvements in delay and power performance.

Characterization of silver nanoparticle inks toward stable roll-to-roll gravure printing

The rheological properties of silver inks are analyzed, and the printing results are presented based on the inks and roll-to-roll printing speed. The shear viscosity, shear modulus, and extensional viscosity of the inks are measured using rotational and extensional rheometers. The inks exhibit the shear thinning power law fluids because the concentration of dispersed nanoparticles in the solvent is sufficiently low, which minimizes elasticity. After the inks are printed on a flexible substrate through gravure printing, the optical images, surface profiles, and electric resistances of the printed pattern are obtained. The width and height of the printed pattern change depending on the ink viscosity, whereas the printing speed does not significantly affect the widening. The drag-out tail is reduced at high ink viscosities and fast printing speeds, thereby improving the printed pattern quality in the roll-to-roll process. Based on the results obtained, we suggest ink and printing conditions that result in high printing quality for complicated printings, such as overlay printing registration accuracy, which imposes pattern widening and drag-out tails in printed patterns.

Design of a Scalable, Flexible, and Durable Thermoelectric Cooling Device for Soft Electronics Using Kirigami Cut Patterns

A flexible, soft thermoelectric cooling device is presented that shows potential for human cooling applications in wearable technologies and close-to-body applications. Current developments lack integration feasibility due to non-scalable assembly procedures and unsuitable materials for comfortable and durable integration into products. Our devices have been created and tested around the need to conform to the human body which we have quantified through the creation of a repeatable drape testing procedure, a metric used in the textile industry. Inspired by mass manufacturing constraints, our flexible thermoelectric devices are created using commercially available materials and scalable processing techniques. Thermoelectric legs are embedded in a foam substrate to provide flexibility, while Kirigami-inspired cuts are patterned on the foam to provide the drape necessary for mimicking the performance of textile and close to body materials. In total, nine different configurations, three different fill factors and three different Kirigami cut patterns were fabricated and inspected for thermal characterization, mechanical testing, flexibility and drape. Our studies show that adding Kirigami patterns can increase the durability of the device, improve the flexibility, decrease the drape coefficient, and have <1% of impact on cooling performance at higher fill factors (>1.5%), reaching temperature differences up to 4.39 ± 0.17°C between the hot and cold faces of the device. These thermoelectric cooling devices show great flexibility, durability, and cooling for integration into soft cooling products.

Nickel Oxide-based Flexible Thin-Film NTC Thermistors by using reverse offset printing

In recent years, the use of printing methods to fabricate electronic devices (printed electronics) has attracted attention because of their low cost and low environmental impact. Printing technology enables the high-throughput fabrication of electrical circuits on film substrates, providing inexpensive personal healthcare devices to monitor health status in real-time. Temperature detection is one of the central concerns as a fundamental physical quantity in various fields. In 2013, a highly sensitive flexible thermistor was reported by formulating aqueous inks of nickel oxide nanoparticles for inkjet printing. However, the calcinating of the nickel oxide (NiO) layer required a high-temperature process of more than 200°C, which required expensive polyimide films with high heat resistance. It is necessary to promote further the development of low-temperature processes for printed thermistors to realize flexible NTC thermistors at low cost using printed electronics technology. In screen printing and inkjet printing, the definition of the ink pattern applied on the substrate changes due to spreading and coffee distortion phenomena, and the thickness between sensors becomes non-uniform, which is a structural consistency problem that can lead to variations in sensing performance. This study developed a printing and low-temperature calcinating method of NTC thermistors with a temperature-sensitive layer of nickel oxide by using reverse offset printing. The NTC thermistors were fabricated by printing a comb-like pattern of silver nanoparticles and a thin nickel oxide film on a glass substrate. In addition, the low-temperature formation of a nickel oxide layer by oxygen plasma treatment was investigated, and XPS was used to carry out compositional analysis of the surface. Together with the plasma-assisted calcinating, a flexible NTC thermistor formed on polyethylene terephthalate (PEN) film is demonstrated.

Flexible, scalable, and efficient thermoelectric touch detector based on PDMS and Graphite flakes

This paper presents freestanding thermoelectric touch detectors consisting of graphite conductive flakes into a polydimethylsiloxane matrix. An optimal concentration of graphite flakes (45 wt%) lead to robust and homogeneous detectors that exhibited signal-noise ratio values up to 170 with rise and falling times below 1 s and 7 s, respectively. The detectors performance was stable over continuous operation and did not reveal significant degradation while bended under different curvature radii (45, 25 and 15 mm) and consecutive bending cycles. Moreover, the twist of the thermal gradient direction between the electrodes of the detector enables a Yes or No response which opens new usage possibilities. Therefore, this work provides an efficient way to develop robust, low-cost, and scalable thermal detectors with potential use in wearable technologies.

Direct ink write multi-material printing of PDMS-BTO composites with MWCNT electrodes for flexible force sensors

With recent advances of additive manufacturing (AM) technology, direct ink write (DIW) printing has allowed to incorporate multi-material printing of various materials with freedom of design and complex geometric shapes to complete functional sensors in a one-step fabrication. This paper introduces the use of DIW 3D printing of polydimethylsiloxane (PDMS) with barium titanate (BTO) filler as stretchable composites with tunable piezoelectric properties that can be used for force sensors applications. To improve the bonding between stretchable piezoelectric composites and electrodes, multi-walled carbon nanotubes (MWCNT) was included in the fabrication of electrodes at a fixed ratio of 11 wt. %. The alignment of the BTO dipoles was achieved through corona poling method, which applies an electric charge on the surface layer of the functional material, aligning the dipoles in the desired direction and thus gaining the piezoelectricity. Different BTO mixing ratios (10-50 wt. %) were evaluated in order to obtain tunable piezoelectric properties and compare the sensitivity with respect their elastic properties. Tensile testing and piezoelectric testing were carried out to characterize mechanical and piezoelectric properties. Results showed that fabricated PDMS with 50 wt. % BTO gave the highest piezoelectric coefficient (d33) of 11.5 pC/N and with an output voltage of 385 mV under compression loading of >200 lbF. This demonstrates feasibility of using multi-material DIW printing to fabricate piezoelectric force sensors with integrated electrodes in one-step without compromising the flexibility of the material.

Towards in-situ quality control of conductive printable electronics: a review of possible pathways

Over the past decade, printed electronics (PE) has shown great potential for a wide range of industries, from consumer goods, electronics, aerospace, automotive, pharmaceutical, biomedical, to textiles and fashion. The rapid development of printing technology has been strongly driven by the growth of the PE market and its many applications. Here, we review the latest trends in PE production quality control, focusing on emerging technologies such as terahertz spectroscopy, which may play a key role in the development of smart manufacturing of PE devices in the near future. We also provide a comparison with conventional quality control technologies or off-line measurements, such as four-point probe measurements, atomic force microscopy, optical microscopy, etc.

Asymmetric electrode geometry induced photovoltaic behavior for self-powered organic artificial synapses

Optoelectronic synapses have attracted considerable attention because of their promising potential in artificial visual perception systems for neuromorphic computing. Despite remarkable progress in mimicking synaptic functions, reduction of energy consumption of artificial synapses is still a substantial obstacle that is required to be overcome to promote advanced emerging applications. Herein, we propose a zero-power artificial optoelectrical synapses using ultrathin organic crystalline semiconductors, which can be self-driven by exploiting the photovoltaic effect induced by asymmetric electrode geometry contacts. The photogenerated charge carrier collection at the two electrodes is unbalanced due to the asymmetric contacts, leading to the in-plane current without bias voltage. Our devices successfully mimic a range of important synaptic functions, such as paired-pulse facilitation (PPF) and spike rate-dependent plasticity (SRDP). Furthermore, we demonstrate that our devices can realize the simulation of image sharpening under self-driven optical-sensing synaptic operations, offering prospects for the development of retinomorphic visual systems.