Nanoscale Bilayer Mechanical Lithography Using Water as Developer.

Sustainability has become a critical concern in the semiconductor industry as hazardous wastes released during the manufacturing process of semiconductor devices have an adverse impact on human beings and the environment. The use of hazardous solvents in existing fabrication processes also restricts the use of polymer substrates because of their low chemical resistance to such solvents. Here, we demonstrate an environmentally friendly mechanical, bilayer lithography that uses just water for development and lift-off. We show that we are able to create arbitrary patterns achieving resolution down to 310 nm. We then demonstrate the use of this technique to create functional devices by fabricating a MoS2 photodetector on a polyethylene terephthalate (PET) substrate with measured response times down to 42 ms.

Soft and ion-conducting hydrogel artificial tongue for astringency perception.

Artificial tongues have been receiving increasing attention for the perception of five basic tastes. However, it is still challenging to fully mimic human tongue-like performance for tastes such as astringency. Mimicking the mechanism of astringency perception on the human tongue, we use a saliva-like chemiresistive ionic hydrogel anchored to a flexible substrate as a soft artificial tongue. When exposed to astringent compounds, hydrophobic aggregates form inside the microporous network and transform it into a micro/nanoporous structure with enhanced ionic conductivity. This unique human tongue-like performance enables tannic acid to be detected over a wide range (0.0005 to 1 wt %) with high sensitivity (0.292 wt %-1) and fast response time (~10 s). As a proof of concept, our sensor can detect the degree of astringency in beverages and fruits using a simple wipe-and-detection method, making a powerful platform for future applications involving humanoid robots and taste monitoring devices.

High-Resolution Filtration Patterning of Silver Nanowire Electrodes for Flexible and Transparent Optoelectronic Devices.

Silver nanowire (AgNW) electrodes attract significant attention in flexible and transparent optoelectronic devices; however, high-resolution patterning of AgNW electrodes remains a considerable challenge. In this study, we have introduced a simple technique for high-resolution solution patterning of AgNW networks, based on simple filtration of AgNW solution on a patterned polyimide shadow mask. This solution process allows the smallest pattern size of AgNW electrodes, down to a width of 3.5 µm. In addition, we have demonstrated the potential of these patterned AgNW electrodes for applications in flexible optoelectronic devices, such as photodetectors. Specifically, for flexible and semitransparent UV photodetectors, AgNW electrodes are embedded in sputtered ZnO films to enhance the photocurrent by light scattering and trapping, which resulted in a significantly enhanced photocurrent (up to 800%) compared to devices based on AgNW electrodes mounted on top of ZnO films. In addition, our photodetector could be operated well under extremely bent conditions (bending radius of approximately 770 µm) and provide excellent durability even after 500 bending cycles.

High-Performance Hybrid Photovoltaics with Efficient Interfacial Contacts between Vertically Aligned ZnO Nanowire Arrays and Organic Semiconductors.

Hybrid photovoltaics (HPVs) incorporating both organic and inorganic semiconducting materials have attracted much attention as next-generation photovoltaics because of their advantage of combining both materials. The hybridization of ZnO nanowires (NWs) and organic semiconductors is expected to be a suitable approach to overcome the limited exciton diffusion length and low electron mobility associated with current organic photovoltaics. The use of ZnO NWs allows researchers to tune nanoscale dimensions more precisely and to achieve rod-to-rod spacing below 10 nm. However, the perfect incorporation of organic semiconductors into densely packed ZnO NW arrays has yet to be achieved. In this study, we report the fabrication of ZnO NW arrays and various organic heterojunction-based HPVs using the feasible and effective vacuum-assisted double coating (VADC) method, achieving full coverage of the organic semiconductors on the compact ZnO NW arrays. The newly proposed VADC method ensures perfect infiltration and full coverage of the organic semiconductors on the densely packed NW arrays. Compared with the conventional single spin-coating process, the use of the VADC method led to 11 and 14% increases in the power conversion efficiency of P3HT:PCBM- and PBDTTT-C-T:PC71BM-based HPVs, respectively. Our studies provide a feasible method for the fabrication of efficient HPVs.

Gate-Tunable and Programmable n-InGaAs/Black Phosphorus Heterojunction Diodes.

Semiconductor heterostructures have enabled numerous applications in diodes, photodetectors, junction field-effect transistors, and memory devices. Two-dimensional (2D) materials and III-V compound semiconductors are two representative materials providing excellent heterojunction platforms for the fabrication of heterostructure devices. The marriage between these semiconductors with completely different crystal structures may enable a new heterojunction with unprecedented physical properties. In this study, we demonstrate a multifunctional heterostructure device based on 2D black phosphorus and n-InGaAs nanomembrane semiconductors that exhibit gate-tunable, photoresponsive, and programmable diode characteristics. The device exhibits clear rectification with a large gate-tunable forward current, which displays rectification and switching with a maximum rectification ratio of 4600 and an on/off ratio exceeding 105, respectively. The device also offers nonvolatile memory properties, including large hysteresis and stable retention of storage charges. By combining the memory and gate-tunable rectifying properties, the rectification ratio of the device can be controlled and memorized from 0.06 to 400. Moreover, the device can generate three different electrical signals by combining a photoresponsivity of 0.704 A/W with the gate-tunable property, offering potential applications, for example, multiple logic operator. This work presents a heterostructure design based on 2D and III-V compound semiconductors, showing unique physical properties for the development of multifunctional heterostructure devices.

Skin-Inspired Hierarchical Polymer Architectures with Gradient Stiffness for Spacer-Free, Ultrathin, and Highly Sensitive Triboelectric Sensors.

The gradient stiffness between stiff epidermis and soft dermis with interlocked microridge structures in human skin induces effective stress transmission to underlying mechanoreceptors for enhanced tactile sensing. Inspired by skin structure and function, we fabricate hierarchical nanoporous and interlocked microridge structured polymers with gradient stiffness for spacer-free, ultrathin, and highly sensitive triboelectric sensors (TESs). The skin-inspired hierarchical polymers with gradient elastic modulus enhance the compressibility and contact areal differences due to effective transmission of the external stress from stiff to soft layers, resulting in highly sensitive TESs capable of detecting human vital signs and voice. In addition, the microridges in the interlocked polymers provide an effective variation of gap distance between interlocked layers without using the bulk spacer and thus facilitate the ultrathin and flexible design of TESs that could be worn on the body and detect a variety of pressing, bending, and twisting motions even in humid and underwater environments. Our TESs exhibit the highest power density (46.7 µW/cm2), pressure (0.55 V/kPa), and bending (∼0.1 V/°) sensitivities ever reported on flexible TESs. The proposed design of hierarchical polymer architectures for the flexible and wearable TESs can find numerous applications in next-generation wearable electronics.

Wearable and flexible sensors for user-interactive health-monitoring devices.

Flexible electronic devices that are lightweight and wearable are critical for personal healthcare systems, which are not restricted by time and space. To monitor human bio-signals in a non-invasive manner, skin-conforming, highly sensitive, reliable, and sustainable healthcare monitoring devices are required. In this review, we introduce flexible and wearable sensors based on engineered functional nano/micro-materials with unique sensing capabilities for detection of physical and electrophysiological vital signs of humans. In addition, we investigate key factors for the development of user-interactive healthcare devices that are customizable, wearable, skin-conforming, and monolithic (design), and have long-term monitoring capability with sustainable power sources. Finally, we describe potential challenges of developing current wearable healthcare devices for applications in fitness, medical diagnosis, prosthetics, and robotics.

High-Performance MoS2/CuO Nanosheet-on-One-Dimensional Heterojunction Photodetectors.

van der Waals heterostructures based on stacked two-dimensional (2D) materials provide novel device structures enabling high-performance electronic and optoelectronic devices. While 2D-2D or 2D-bulk heterostructures have been largely explored for fundamental understanding and novel device applications, 2D-one-dimensional (1D) heterostructures have been rarely studied because of the difficulty in achieving high-quality heterojunctions between 2D and 1D structures. In this study, we introduce nanosheet-on-1D van der Waals heterostructure photodetectors based on a wet-transfer printing of a MoS2 nanosheet on top of a CuO nanowire (NW). MoS2/CuO nanosheet-on-1D photodetectors show an excellent photocurrent rectification ratio with an ideality factor of 1.37, which indicates the formation of an atomically sharp interface and a high-quality heterojunction in the MoS2/CuO heterostructure by wet-transfer-enhanced van der Waals bonding. Furthermore, nanosheet-on-1D heterojunction photodetectors exhibit excellent photodetection capabilities with an ultrahigh photoresponsivity (∼157.6 A/W), a high rectification ratio (∼6000 at ±2 V), a low dark current (∼38 fA at -2 V), and a fast photoresponse time (∼34.6 and 51.9 ms of rise and decay time), which cannot be achievable with 1D-on-nanosheet heterojunction photodetectors. The wet-transfer printing of nanosheet-on-1D heterostructures introduced in this study provides a robust platform for the fundamental study of various combinations of 2D-on-1D heterostructures and their applications in novel heterojunction devices.

InGaAs Nanomembrane/Si van der Waals Heterojunction Photodiodes with Broadband and High Photoresponsivity.

Development of broadband photodetectors is of great importance for applications in high-capacity optical communication, night vision, and biomedical imaging systems. While heterostructured photodetectors can expand light detection range, fabrication of heterostructures via epitaxial growth or wafer bonding still faces significant challenges because of problems such as lattice and thermal mismatches. Here, a transfer printing technique is used for the heterogeneous integration of InGaAs nanomembranes on silicon semiconductors and thus the formation of van der Waals heterojunction photodiodes, which can enhance the spectral response and photoresponsivity of Si photodiodes. Transfer-printed InGaAs nanomembrane/Si heterojunction photodiode exhibits a high rectification ratio (7.73 × 104 at ±3 V) and low leakage current (7.44 × 10-5 A/cm2 at -3 V) in a dark state. In particular, the photodiode shows high photoresponsivities (7.52 and 2.2 A W-1 at a reverse bias of -3 V and zero bias, respectively) in the broadband spectral range (400-1250 nm) and fast rise-fall response times (13-16 ms), demonstrating broadband and fast photodetection capabilities. The suggested III-V/Si van der Waals heterostructures can be a robust platform for the fabrication of high-performance on-chip photodetectors compatible with Si integrated optical chips.

Octopus-Inspired Smart Adhesive Pads for Transfer Printing of Semiconducting Nanomembranes.

By mimicking muscle actuation to control cavity-pressure-induced adhesion of octopus suckers, smart adhesive pads are developed in which the thermoresponsive actuation of a hydrogel layer on elastomeric microcavity pads enables excellent switchable adhesion in response to a thermal stimulus (maximum adhesive strength: 94 kPa, adhesion switching ratio: ≈293 for temperature change between 22 and 61 °C).

Vacuum-induced wrinkle arrays of InGaAs semiconductor nanomembranes on polydimethylsiloxane microwell arrays.

Tunable surface morphology in III-V semiconductor nanomembranes provides opportunities to modulate electronic structures and light interactions of semiconductors. Here, we introduce a vacuum-induced wrinkling method for the formation of ordered wrinkles in InGaAs nanomembranes (thickness, 42 nm) on PDMS microwell arrays as a strategy for deterministic and multidirectional wrinkle engineering of semiconductor nanomembranes. In this approach, a vacuum-induced pressure difference between the outer and inner sides of the microwell patterns covered with nanomembranes leads to bulging of the nanomembranes at the predefined microwells, which, in turn, results in stretch-induced wrinkle formation of the nanomembranes between the microwells. The direction and geometry of the nanomembrane wrinkles are well controlled by varying the PDMS modulus, depth, and shape of microwells, and the temperature during the transfer printing of nanomembrane onto heterogeneous substrates. The wrinkling method shown here can be applied to other semiconductor nanomembranes and may create an important platform to realize unconventional electronic devices with tunable electronic properties.

Gate-controlled spin-orbit interaction in inas high-electron mobility transistor layers epitaxially transferred onto Si substrates.

We demonstrate gate-controlled spin-orbit interaction (SOI) in InAs high-electron mobility transistor (HEMT) structures transferred epitaxially onto Si substrates. Successful epitaxial transfer of the multilayered structure after separation from an original substrate ensures that the InAs HEMT maintains a robust bonding interface and crystalline quality with a high electron mobility of 46200 cm(2)/(V s) at 77 K. Furthermore, Shubnikov-de Haas (SdH) oscillation analysis reveals that a Rashba SOI parameter (α) can be manipulated using a gate electric field for the purpose of spin field-effect transistor operation. An important finding is that the α value increases by about 30% in the InAs HEMT structure that has been transferred when compared to the as-grown structure. First-principles calculations indicate that the main causes of the large improvement in α are the bonding of the InAs HEMT active layers to a SiO2 insulating layer with a large band gap and the strain relaxation of the InAs channel layer during epitaxial transfer. The experimental results presented in this study offer a technological platform for the integration of III-V heterostructures onto Si substrates, permitting the spintronic devices to merge with standard Si circuitry and technology.

Fabrication of uniform layer-by-layer assemblies with complementary protein cage nanobuilding blocks via simple His-tag/metal recognition.

A capsid-forming enzyme, lumazine synthase isolated from hyperthermophile Aquifex aeolicus (AaLS), is prepared and utilized as a template for constructing nanobuilding blocks to fabricate uniform layer-by-layer (LbL) assemblies. Two functionally complementary AaLS protein cage nanoparticles (PCNs) are generated either by genetically introducing His-tags on the surface of wild-type AaLS PCNs or by chemically attaching metal chelates (Ni-NTA moiety) to the surface of cysteine-bearing AaLS PCNs individually. The multivalent displays of His-tags (AaLS-His6 PCN) and Ni-NTA ligands (AaLS-NTA-Ni PCN) on the surface of each complementary AaLS PCN are successfully demonstrated by mass spectrometric and surface plasmon resonance analyses. By using these two complementary AaLS PCNs, uniform LbL assemblies are constructed via simple recognition between His-tags and metal chelates without the aid of additional binding mediators. This approach illustrates the potential of fabricating uniform nanostructures using protein-based hybrid functional nanobuilding blocks.