Bhupendra Kumar Sharma

Stretchable alternating-current electroluminescent (ACEL) devices are required due to their potential in wearable, biomedical, e-skin, robotic, lighting, and display applications; however, one of the main hurdles is achieving uniform electroluminesence, demanding an optimal combination of transparency, conductivity, and stretchability in electrodes. We therefore propose a fabrication scheme involving strategically combining two-dimensional graphene layers with a silver nanowires (Ag NWs) embedded PEDOT:PSS film. The developed hybrid electrode overcomes the limitations of commonly known metallic NWs and ionic conductor-based electrodes for ACEL applications. Furthermore, the potential of the hybrid electrode is realized in demonstrating large-area stretchable ACEL devices composed of an 8 × 8 passive array. The prototype ACEL passive array demonstrates efficient and uniform electroluminescence under high levels of mechanical deformation such as bending, rolling, twisting, and stretching.

Oxide semiconductors typically show superior device performance compared to amorphous silicon or organic counterparts, especially when they are physical vapor deposited. However, it is not easy to reproduce identical device characteristics when the oxide field-effect transistors (FETs) are solution-processed/printed; the level of complexity further intensifies with the need to print the passive elements as well. Here, we developed a protocol for designing the most electronically compatible electrode/channel interface based on the judicious material selection. Exploiting this newly developed fabrication schemes, we are now able to demonstrate high-performance all-printed FETs and logic circuits using amorphous indium–gallium–zinc oxide (a-IGZO) semiconductor, indium tin oxide (ITO) as electrodes, and composite solid polymer electrolyte as the gate insulator. Interestingly, all-printed FETs demonstrate an optimal electrical performance in terms of threshold voltages and device mobility and may very well be compared with devices fabricated using sputtered ITO electrodes. This observation originates from the selection of electrode/channel materials from the same transparent semiconductor oxide family, resulting in the formation of In–Sn–Zn–O (ITZO)-based-diffused a-IGZO–ITO interface that controls doping density while ensuring high electrical performance. Compressive spectroscopic studies reveal that Sn doping-mediated excellent band alignment of IGZO with ITO electrodes is responsible for the excellent device performance observed. All-printed n-MOS-based logic circuits have also been demonstrated toward new-generation portable electronics.

Efficient and highly functional three-dimensional systems that are ubiquitous in biology suggest that similar design architectures could be useful in electronic and optoelectronic technologies, extending their levels of functionality beyond those achievable with traditional, planar two-dimensional platforms. Complex three-dimensional structures inspired by origami, kirigami have promise as routes for two-dimensional to three-dimensional transformation, but current examples lack the necessary combination of functional materials, mechanics designs, system-level architectures, and integration capabilities for practical devices with unique operational features. Here, we show that two-dimensional semiconductor/semi-metal materials can play critical roles in this context, through demonstrations of complex, mechanically assembled three-dimensional systems for light-imaging capabilities that can encompass measurements of the direction, intensity and angular divergence properties of incident light. Specifically, the mechanics of graphene and MoS₂, together with strategically configured supporting polymer films, can yield arrays of photodetectors in distinct, engineered three-dimensional geometries, including octagonal prisms, octagonal prismoids, and hemispherical domes.

Highly flexible organic light-emitting diode display was demonstrated using MoS ₂ TFTs as an active-matrix backplane.

The transfer of synthesized large‐area 2D materials to arbitrary substrates is expected to be a vital step for the development of flexible device fabrication processes. The currently used hazardous acid‐based wet chemical etching process for transferring large‐area MoS₂ films is deemed to be unsuitable because it significantly degrades the material and damages growth substrates. Surface energy‐assisted water‐based transfer processes do not require corrosive chemicals during the transfer process; however, the concept is not investigated at the wafer scale due to a lack of both optimization and in‐depth understanding. In this study, a wafer‐scale water‐assisted transfer process for metal–organic chemical vapor‐deposited MoS₂ films based on the hydrofluoric acid treatment of a SiO₂ surface before the growth is demonstrated. Such surface treatment enhances the strongly adhering silanol groups, which allows the direct transfer of large‐area, continuous, and defect‐free MoS₂ films; it also facilitates the reuse of growth substrate. The developed transfer method allows direct fabrication of flexible devices without the need for a polymeric supporting layer. It is believed that the proposed method can be an alternative defect‐ and residue‐free transfer method for the development of MoS₂‐based next‐generation flexible devices.

Electronic applications that offer flexibility and stretchability have attracted increasing interest over the past few years because of their potential applications in, for example, electronic skin and bio‐inspired devices, where electronics based on current wafer‐based technology are not suitable. Metal‐oxide‐based binary and ternary systems offer the opportunity for large‐area uniform synthesis of materials with excellent electrical performance. This review mainly focuses on basic flexibility and stretchability concepts that can be integrated and applied to all inorganic‐material‐based electronic devices. Possible fabrication methods are also discussed, and oxide‐based electronic applications summarized in this context. Finally, the review is concluded with a discussion of future prospects of oxides in flexible and stretchable devices.

The instability of an amorphous indium–gallium–zinc oxide (IGZO) field effect transistor is investigated upon water treatment. Electrical characteristics are measured before, immediately after and a few days after water treatment in ambient as well as in vacuum conditions. It is observed that after a few days of water exposure an IGZO field effect transistor (FET) shows relatively more stable behaviour as compared to before exposure. Transfer characteristics are found to shift negatively after immediate water exposure and in vacuum. More interestingly, after water exposure the off current is found to decrease by 1–2 orders of magnitude and remains stable even after 15 d of water exposure in ambient as well as in vacuum, whereas the on current more or less remains the same. An x-ray photoelectron spectroscopic study is carried out to investigate the qualitative and quantitative analysis of IGZO upon water exposure. The changes in the FET parameters are evaluated and attributed to the formation of excess oxygen vacancies and changes in the electronic structure of the IGZO bulk channel and at the IGZO/SiO ₂ interface, which can further lead to the formation of subgap states. An attempt is made to distinguish which parameters of the FET are affected by the changes in the electronic structure of the IGZO bulk channel and at the IGZO/SiO ₂ interface separately.

We demonstrate photo-patternable ion gel-gated graphene transistors and inverters on plastic substrates. The photo-patternable ion gel can be used as a negative photoresist for the patterning of underlying graphene as well as gate dielectrics. As a result, an extra graphene-patterning step is not required, which simplifies the device fabrication and avoids a side effect arising from the photoresist residue. The high capacitance of ion gel gate dielectrics yielded a low voltage operation (∼2 V) of the graphene transistor and inverter. The graphene transistors on plastic showed an on/off-current ratio of ∼11.5, along with hole and electron mobilities of 852 ± 124 and 452 ± 98 cm2 V−1 s−1, respectively. In addition, the flexible graphene inverter was successfully fabricated on plastic through the potential superposition effect from the drain bias. These devices show excellent mechanical flexibility and fatigue stability.