Bioelectrochemical Systems

Owing to excellent performance and a simple mechanism, stretchable piezoresistive strain sensors have been applied to human skin for monitoring physical activities, physiological activities, etc. However, it is still a challenge to simultaneously realize highly sensitive and stretchable piezoresistive strain sensors with high optical transparency. This study reports a transparent and stretchable piezoresistive strain sensor with 2D surface buckling by fabricating ultrathin indium tin oxide (ITO) film on the biaxially pre-stretched polyacrylate (VHB) elastomer followed by pre-stretch releasing. To the authors' knowledge, semiconductors are applied for a stretchable piezoresistive strain sensor for the first time. Furthermore, this strain sensor exhibits a high sensitivity of 569, a high transparency of 88% and a high biaxial stretchability of 110% at the same time. This device demonstrates the good long-term stability over 500 stretching–relaxing cycles. The high sensitivity can be mainly attributed to the piezoresistive effect of the semiconductor where carrier mobility and the resulting resistivity can be significantly changed by the strain. The strain sensors attached to human skin are used to monitor many human motions such as chewing, swallowing, breathing, and walking. ITO-based strain sensors pave the way toward the development of highly stretchable and sensitive wearable electronics.

In the quest to reduce energy consumption, there is a growing demand for technology beyond silicon as electronic materials for neuromorphic artificial intelligence devices. Equipped with the criteria of energy efficiency and excellent adaptability, organohalide perovskites can emulate the characteristics of synaptic functions in the human brain. In this aspect, this study designs and develops CsFAPbI3-based memristive neuromorphic devices that can switch at low power and show larger endurance by adopting the powder engineering methodology. The neuromorphic characteristics of the CsFAPbI3-based devices exhibit an ultra-high paired-pulse facilitation index for an applied electric stimuli pulse. Moreover, the transition from short-term to long-term memory requires ultra-low energy with long relaxation times. The learning and training cycles illustrate that the CsFAPbI3-based devices exhibit faster learning and memorization process owing to their larger carrier lifetime compared to other perovskites. The results provide a pathway to attain low-power neuromorphic devices that are synchronic to the human brain's performance.

Hybrid two-dimensional materials consisting of graphene and hexagonal boron nitride (h-BN) have drawn significant interest due to their tunable bandgap and electrical properties. Considering their composition-dependent properties, ohmic current injection and the development of h-BN-based optoelectronic and high-power electronic devices should be achievable by controlling the C concentration. In this study, electrical and optical characterizations of single-crystal h-BN synthesized under high-pressure and high-temperature (HPHT) are conducted by varying C concentrations via post-growth diffusion. Low C-doped h-BN (h-BN:C) with ≈0.1 at% C exhibits nonohmic conduction within a voltage range of ±100 V at all temperatures. In contrast, high h-BN:C (≈10 at% C) containing C domains and graphite/graphene layers shows additional luminescence peaks and initially exhibits nonohmic conduction at 298 K, which then transforms to ohmic conduction after breakdown-like behavior at 598 K. This phenomenon, observed only in the high h-BN:C devices, is attributed to the C-containing conductive path formed on the channel surface through C drift and local dielectric breakdown of h-BN mother phase, indicating that ohmic conduction itself does not guarantee the current flow in the conduction/valence bands in h-BN:C. With these findings, the present thorough and fruitful characterizations are beneficial for the development of h-BN:C-based devices.

Janus nanosystems enable one to achieve complementary properties in a single entity. In the current study, the fundamental properties like structural, electronic, and dynamical of Janus hexagonal boron nitride (h-BN) by selectively hydrogenating and fluorinating a h-BN surface are systematically examined, using density functional theory. Functionalization of h-BN introduces partial sp3 (buckled) character in the predicted materials as compared to planar sp2 h-BNs. Fully fluorinated and hydrogenated h-BN have a direct bandgap of 3.42 and 3.37 eV, respectively. All the investigated configurations are predicted to be dynamically stable. Furthermore, optical properties including dielectric function, absorption spectra, refractive index, and reflectivity are evaluated to realize the optical and photocatalytic behavior of considered systems. The dielectric function ɛ2(ω) shows fundamental absorption edge arising at 3.2, 3.9, 2.8, and 3.4 eV for hydrogen on boron and nitrogen, hydrogen on boron and fluorine on nitrogen, fluorine on boron and hydrogen on nitrogen (FBNH) and fluorine on boron and nitrogen which is comparable to the bandgap of respective monolayers. Solar cell parameters of all considered BN structures are calculated using the Shockley–Queisser (SQ) limit. The highest short-circuit current density (Jsc ) for FBNH is found to be 2.1 mA cm−2 providing the efficiency of 8.27% making FBNH a potential candidate for absorber layer in solar cells.

The immense increase of unstructured data require novel computing systems that can process the input data with low power and parallel processing. This functionality is similar to that of human brains that are composed of numerous neurons, synapses, and their complex connections. To mimic the functionality of the human brain with an electronic device, the resistive switching device and crossbar array has attracted considerable attention for artificial synaptic devices and integrated systems, respectively. For this purpose, the self-rectifying resistive switching cell based on the Si:ZrOx thin film is developed and its reliability characteristics are tested. Four achievements are highlighted in this study. 1) The retention characteristic is improved by the adoption of TaOx thin film as an oxygen reservoir layer. 2) The asymmetric electrodes can make the self-rectifying resistive cell (SRC) have sufficient rectifying characteristic. 3) The linearity of conductance update has a dominant effect on the inference performance compared to that of the conductance range variation. 4) The device of the interface-type resistive switching shows a high enough device yield in the crossbar array device and exhibits reliable multiply-and-accumulate operations in the crossbar array to mimic the human brain-inspired computing system.

Unbalanced mobility and injection of charge carriers in metal-halide perovskite light-emitting devices pose severe limitations to the efficiency and response time of the electroluminescence. Modulation of gate bias in methylammonium lead iodide light-emitting transistors has proven effective in increasing the brightness of light emission up to MHz frequencies. In this work, a new approach is developed to improve charge carrier injection and enhance electroluminescence of perovskite light-emitting transistors by independent control of drain–source and gate–source bias voltages to compensate for space-charge effects. Optimization of bias pulse synchronization induces a fourfold enhancement of the emission intensity. Interestingly, the optimal phase delay between biasing pulses depends on modulation frequency due to the capacitive nature of the devices, which is well captured by numerical simulations of an equivalent electrical circuit. These results provide new insights into the electroluminescence dynamics of AC-driven perovskite light-emitting transistors and demonstrate an effective strategy to optimize device performance through independent control of the amplitude, frequency, and phase of the biasing pulses.

Optoelectronic sampling is the ultimate method to perform ultra-high frequency analog-to-digital conversion. Thanks to the ps photo-thermo-bolometric effect in high mobility hexagonal boron nitride (h-BN) encapsulated graphene, a state-of-the-art optoelectronic sampler over a 40 GHz bandwidth using a 4 ps pulsed laser is demonstrated. The superior transport and optoelectronic linearities of this graphene sampler lead to very high harmonic rejections below –43 dB above 20 GHz. With a physics-based microscopic model, it is shown that harmonics are generated by optoelectronics saturation effects and only appear when carrier energy reaches that of optical phonons, which is very high in h-BN encapsulated graphene. This device is the first ultra-fast optoelectronic sampler based on the bolometric effect.

High-performance, coplanar amorphous In0.5Ga0.5O (a-IGO) thin film transistor (TFT) on a polyimide (PI) substrate deposited by spray pyrolysis (SP) is reported. The SP a-IGO film deposited at 370 °C has less than 8% nanocrystalline-In2O3 dots and a mass density of 6.6 g cm−3. The a-IGO TFT on PI exhibits linear mobility over 30 cm2 V−1 s−1 and a negligible shift in threshold voltage (ΔVTH = 2.3 MHz) with a low propagation delay time of ≈30 ns per stage at a supply voltage (VDD) of 15 V. The fabricated gate shift register circuit works up to the last stage without any decrement of the input voltage (15 V) with rising and falling times less than 0.8 µs. Therefore, the coplanar a-IGO TFT by SP deposited at 370 °C is a promising candidate for low-cost, flexible TFT backplanes of foldable electronics.

Semiconducting transition metal dichalcogenides (TMDC) are 2D materials, combining good charge carrier mobility, ultimate dimension down-scalability, and low-temperature integration. These properties make TMDCs interesting for flexible electronics, where the thermal fabrication budget is strongly substrate limited. In this perspective, an overview of the state of TMDC research is provided by evaluating two scenarios, both with their own merit depending on the target application. First, high-quality chemically grown 2D TMDCs are promising for nanoscale high-performance and high-frequency devices with excellent gate control and high current on/off ratios. Second, TMDC thin films can also be solution deposited from chemically exfoliated flakes allowing for moderate performance, but providing a path toward low-cost production. A strong advantage of TMDCs is the possibility to realize p-type and n-type channels for complementary transistors having similar performance figures-of-merit. This aspect, as well as common transistor performance metrics are also compared with other flexible channel materials providing an overview of the state of the art of thin-film transistors in the field of flexible electronics.

The development of field-effect transistor-based (FET-based) non-volatile optoelectronic memories is vital toward innovations necessary to improve computer systems. In this work, for the first time, the unique charge-trapping and charge-retention properties of solution-processed colloidal nitrogen-doped carbon quantum dots (CQDs) are harnessed to achieve functional optoelectronic memories programmable by UV illumination with a multilevel writing possibility. Of particular note, long-lasting memory function can be achieved thanks to the vast charge trapping sites provided by the N-doped CQDs and the resultant photo-gating effect is exercised on the graphene FET. The achieved memory can be erased by a positive gate bias which provides sufficient carriers to remove trapped charges through recombination. This study highlights the possibility to engineer high-performance all-carbon non-volatile FET-based optoelectronic memories through manipulating and coupling the charge-trapping properties of colloidal CQDs and graphene.

The flexible type of displays, which can alter their shape freely (e.g., bendable or foldable displays) according to situational demands, is under an intense spotlight due to applications in human-friendly mobile electronics. Among various types of light-emitting diodes (LEDs), meanwhile, perovskite-based LEDs (PeLEDs) have garnered particular attention in recent years as next-generation optoelectronic devices due to their exceptional optical characteristics, such as high efficiency (up to theoretical limits), high color purity, wide color gamut over the entire visible range, and ultrathin form factor. By integrating perovskite materials on an ultrathin flexible substrate with a proper encapsulation layer, the flexible PeLEDs, which can conform to various curved surfaces and be stably operated in an ambient condition, have been developed. Here, recent advances of the flexible PeLEDs are reviewed. First, light-emitting materials and device structures for the PeLEDs are introduced. Then, research progress on the flexible PeLEDs, either with and without the encapsulation layer are summarized. Next, some representative application examples of the flexible PeLEDs are briefly discussed. Finally, this review includes a brief discussion on future prospects.

With the fast growth of the number of electronic devices on the internet of things (IoT), hardware-based security primitives such as physically unclonable functions (PUFs) have emerged to overcome the shortcomings of conventional software-based cryptographic technology. Existing PUFs exploit manufacturing process variations in a semiconductor foundry technology. This results in a static challenge–response behavior, which can present a long-term security risk. This study shows a reconfigurable PUF based on nanoscale magnetic tunnel junction (MTJ) arrays that uses stochastic dynamics induced by voltage-controlled magnetic anisotropy (VCMA) for true random bit generation. A total of 100 PUF instances are implemented using 10 ns voltage pulses on a single chip with a 10 × 10 MTJ array. The unipolar nature of the VCMA mechanism is exploited to stabilize the MTJ state and eliminate bit errors during readout. All PUF instances show entropy close to one, inter-Hamming distance close to 50%, and no bit errors in 104 repeated readout measurements.

Due to the complicated relationship between wearable electronics performance and various parameters, months even years are needed to obtain a desired sensor by random and time-consuming trial-and-error methods. Herein, a general analytic model based on the micro-element division and equivalent circuit is presented to guide a rapid optimizing strategy for wearable resistive pressure sensors, which is like the method always used in the traditional design of the metal-oxide-semiconductor field-effect transistor for integrated circuits. The quantitative relationship between the sensitivity and related parameters is declared in the presented model, and the optimized parameters are achieved to design a sensor. The demanded ultra-highly sensitive pressure sensor is successfully designed and optimized in minutes based on the built model, and the fabricated sensor is applied in a voice real-time recognition system to obtain 100% recognition accuracy. The on-demand and agile development strategy paves a promising way to greatly accelerate the transition from random and time-consuming to the controllable design of wearable electronics.

Gas sensors are useful for monitoring of greenhouse gases. As the move toward complementary metal-oxide-semiconductor (CMOS) compatible pyroelectric room temperature detectors is gaining traction due to its scalability to 8-in./12-in. wafer area and integrable with CMOS electronics, CMOS compatible aluminum nitride (AlN)- and scandium aluminum nitride (ScAlN)-based pyroelectric detectors are developed for sensing of CO2and CH4gases, which are two of the greenhouse gases that contribute significantly to global warming. Leveraging gas absorption at respective mid-infrared (IR) wavelengths, CO2 and CH4 gases are detected at various concentrations with fast response time ≈1 s. A compound parabolic collector (CPC) is designed and integrated into the gas sensor to enhance the optical flux received by the detector, which demonstrates ≈10× signal improvement in its presence. Further factors such as the effect of sensing area reduction and response to random gas concentrations are also tested on AlN- and ScAlN-based pyroelectric detectors respectively to observe the gas sensing behaviors of both detectors. The results obtained provide further understanding of the behavior of CMOS AlN- and ScAlN-based pyroelectric detectors as IR gas sensors, which can potentially inspire new design and design selection for various gas sensing applications.

The 2D GeSe-based photodetectors have exhibited ultrahigh photoresponsivity (Rλ), sensitive-specific detectivity (D*), and large external quantum efficiency (EQE) in previous researches. Ion beam techniques have been utilized to effectively modify the surface of nanomaterials for recent years. Herein, the authors propose to engineer the 2D GeSe nanosheets via low-energy ion irradiation for improving the photoresponse in visible region. The nonmetallic nitrogen and metallic silver elements are selected to modulate the performance of 2D GeSe FETs, respectively. The results show that N-irradiated GeSe nanosheets have exhibited twice faster photoresponse for 532 nm laser for making up the trailing phenomenon in the decay process during the dynamic response of pristine 2D GeSe. More importantly, via Ag ion irradiation, a self-driven and higher photoresponsivity GeSe-based photodetector is realized. The Ag-irradiated GeSe nanosheets have shown the considerably high Rλ of 9.6 × 102 A W−1 with no bias and no gate voltage applied. The work provides a new direction for modification of other 2D materials by ion beam technique for optoelectronic devices.

Carbon, especially graphene quantum dots (GQDs) based electronics have become an attractive technology in recent years. The controlled modification of the electrical and optoelectronic properties of GQDs by physical/chemical processes or synthetic methods may lead to new applications. Gadolinium-doped polyethyleneimine (PEI) functionalized and nitrogen-doped graphene quantum dots (GdNPs-PEI@N-GQDs) are synthesized by a hydrothermal method to determine how doping carbon-based materials with Gd alters the electrical properties of the structure. The electrical properties of the GdNPs/PEI@N-GQDs nanocomposite-based diode are investigated using the current–voltage (I–V) technique and the capacitance and conductance voltage (C–V & G/ω–V) technique at 300 K in the frequency range of 0.5 to 500 kHz at ± 5 V. The rectification ratio (RR) is found to be 14 at a voltage of ±5 V. The rectifying behavior of the diode changes to an ohmic behavior after doping with Gd, compared to the Gd-free PEI@N-GQDs sample (2.8 × 104 at ±5 V). The results are expected to have an impact on the understanding of carbon-based electronics technology.

Memory devices are an essential part of modern electronics. Efforts to move beyond the traditional “read” and “write” of digital information in volatile and non-volatile memory devices are leading to the rapid growth of neuromorphic technology. There is a growing demand for memory devices with continuous memory states with various retention times and greater integration density with more energy-efficient mechanisms. Two types of memory devices (i.e., non-volatile digital memory and neuro-synaptic devices) have been extensively investigated with emerging materials. Among numerous materials for such memory devices, in this review, the authors focus on 2D ferroelectric materials for promising memory and synaptic devices. Three types of memory devices based on 2D ferroelectric materials are classified and discussed here: 1) ferroelectric gating of semiconducting channels, 2) active ferroelectric channels, and 3) ferroelectric tunnel junction devices. It is known that atomically thin geometry competes with ferroelectricity, which can degrade the quality of the devices based on atomically thin ferroelectric materials. Various efforts to resolve the fundamental issue with emerging 2D ferroelectric materials and how they can be used as a critical element for memory and synaptic devices are surveyed.

2D transition-metal dichalcogenide (TMDC) materials are promising candidates with excellent thermoelectric (TE) properties owing to their low dimensionality in electronic and phonon transport. However, the considerable coupling of the Seebeck coefficient and electrical conductivity in such TE materials eventually results in the limit of the TE power factor increase, which severely hinders potential TE device applications. Herein, an alternative approach is demonstrated for breaking the strong coupling between the Seebeck coefficient and electrical conductivity in single TE materials by adopting a novel stacked PtSe2/PtSe2 homostructure. By alternately piling low-resistance (LR) PtSe2 (3 nm) onto high-resistance (HR) PtSe2 (2 nm) as one unit, the Seebeck coefficient and electrical conductivity of such stacked homostructures can be greatly enhanced with slightly improved electrical conductivity, ultimately resulting in a TE power factor in three-unit-stacked homostructures that is ≈1,648% higher than that of a single PtSe2 (15 nm) layer with the same thickness. This enhancement is attributed to an independent increase in the Seebeck coefficient, which depends on the interface among the LR and HR PtSe2 layers. The findings pave the way for a method that, unlike power factor optimization in conventional thermoelectric materials, can only utilize the Seebeck coefficient and electrical conductivity of each layer in a stacked homostructure.

Charged Domain Walls