Achieving Room Temperature Operation Research Articles

Polypyrrole-Tungsten Oxide Nanocomposite Fabrication through Laser-Based Techniques for an Ammonia Sensor: Achieving Room Temperature Operation.

A highly sensitive ammonia-gas sensor based on a tungsten trioxide and polypyrrole (WO3/PPy) nanocomposite synthesized using pulsed-laser deposition (PLD) and matrix-assisted pulsed-laser evaporation (MAPLE) is presented in this study. The WO3/PPy nanocomposite is prepared through a layer-by-layer alternate deposition of the PPy thin layer on the WO3 mesoporous layer. Extensive characterization using X-ray diffraction, FTIR and Raman spectroscopy, scanning electron microscopy, atomic force microscopy, and water contact angle are carried out on the as-prepared layers. The gas-sensing properties of the WO3/PPy nanocomposite layers are systematically investigated upon exposure to ammonia gas. The results demonstrate that the WO3/PPy nanocomposite sensor exhibits a lower detection limit, higher response, faster response/recovery time, and exceptional repeatability compared to the pure PPy and WO3 counterparts. The significant improvement in gas-sensing properties observed in the WO3/PPy nanocomposite layer can be attributed to the distinctive interactions occurring at the p-n heterojunction established between the n-type WO3 and p-type PPy. Additionally, the enhanced surface area of the WO3/PPy nanocomposite, achieved through the PLD and MAPLE synthesis techniques, contributes to its exceptional gas-sensing performance.

Enhanced H2 sensing performance of Pd-modified ZnO nanorods under visible light excitation

High-performance room-temperature hydrogen (H2) sensors are the optimal choice for practical applications. Light excitation, especially visible light, is safer and requires less power compared to ultraviolet (UV) light. This makes it an effective method for achieving room temperature operation. In this study, different mass ratios of Pd-modified ZnO nanorods were synthesized using a combination of hydrothermal and impregnation methods, and their sensing performance to H2 under visible light excitation was investigated. Material characterization results demonstrated that the localized surface plasmon resonance (LSPR) effect of Pd broadened the absorption in the visible light region, increasing the generation of high-energy hot electrons. Notably, the 2.5 wt% Pd/ZnO exhibited excellent sensing performance towards 5000 ppm H2 at room temperature under visible light excitation, with a high response value of 5906.78. The LSPR effect and the catalytic activity of Pd nanoparticles significantly enhance the sensing performance of H2. Based on our current knowledge, there have been no explicit reports on the detection of H2 using visible light excitation with Pd/ZnO, thus highlighting a new direction for the study of room temperature H2 sensors.

High-Performance Optoelectronic Gas Sensing Based on All-Inorganic Mixed-Halide Perovskite Nanocrystals with Halide Engineering.

Gas sensors are of great interest to portable and miniaturized sensing technologies with applications ranging from air quality monitoring to explosive detection and medical diagnostics, but the existing chemiresistive NO2 sensors still suffer from issues such as poor sensitivity, high operating temperature, and slow recovery. Herein, a high-performance NO2 sensors based on all-inorganic perovskite nanocrystals (PNCs) is reported, achieving room temperature operation with ultra-fast response and recovery time. After tailoring the halide composition, superior sensitivity of ≈67 at 8ppm NO2 is obtained in CsPbI2 Br PNC sensors with a detection level down to 2 ppb, which outperforms other nanomaterial-based NO2 sensors. Furthermore, the remarkable optoelectronic properties of such PNCs enable dual-mode operation, i.e., chemiresistive and chemioptical sensing, presenting a new and versatile platform for advancing high-performance, point-of-care NO2 detection technologies.

All-optical transistor using deep-level defects in nitride semiconductors for room temperature optical computing

The essential device for optical computing is an all-optical transistor in which a weak “gate” light controls the strong “source” light. Particularly promising for application in logic operations are all-optical transistors using quasiparticles in a semiconductor because they can be easily integrated into circuits in a way similar to that of conventional electronic ones. However, the practical development of such devices has so far been limited due to extreme difficulties in achieving room temperature operation. In this work, we proposed and numerically verified a scheme of the high-temperature stable all-optical transistor, where light controls light by using deep-level defects in non-polar InGaN/GaN heterostructure and photo-exited holes as an intermediate medium. The developed optical switching concept fulfills all criteria for the useful all-optical transistor listed in Miller, Nat. Photonics 4, 3 (2010), in particular fan-out and cascadability, which are the most difficult to meet. For the design of our transistor, we applied an entirely new approach to III-nitride device physics: we turned usually undesirable deep-level defects into a key, active element of the transistor in which they realize on and off operations. Due to this, the developed device was able to obtain excellent operation stability in a wide temperature range up to 500 K as well as an extremely high on/off ratio (106) and gain (100). Finally, in order to show that the proposed transistor concept is feasible, we performed the gated-photoluminescence experiment for metal–oxide–semiconductor GaN structures.

Enhancement of Low-Temperature Gas-Sensing Performance Using Substoichiometric WO3-x Modified with CuO.

To verify the effect of oxygen vacancy on gas sensitivity, we have systematically investigated the gas-sensing performance of copper oxide/substoichiometric tungsten oxide (CuO/WO3-x) nanocomposite sensors. Oxygen deficiency in WO3-x facilitates the reaction of hydrogen sulfide (H2S) gas with chemisorbed oxygen species (i.e., O2-, O-, and O2-) at low temperature. The oxygen/sulphur exchange reaction between CuO and H2S in the sensing process can achieve room temperature operation of gas sensors. After the WO3-x nanorods were modified by a low content of CuO nanoparticles (Cu:W = 1:20), the sensors present an n-type sensing behavior. Their best working temperatures drop from 289 °C (or 386 °C) to 99 °C (or 70 °C) at which the responses are improved by 14 to 163 times for different x values. Among them, CuO(L)/W5O14 shows the highest sensitivity of 1575.7 to 10 ppm H2S at 99 °C and 171.5 to 10 ppm H2S at room temperature. Once WO3-x were loaded with a high concentration of CuO nanoparticles (Cu:W = 1:2), they exhibit a p-type behavior, and the optimal working temperatures reduce suddenly to room temperature at which CuO(H)/W18O49 displays the most sensitive response of 7.2 even toward trace amounts of H2S as low as 100 ppb. In addition, p-type CuO weakens the metal-like characteristics of W18O49 and such weakening effect enhances with an increase in the CuO content. Therefore, the sensing performance of the CuO/W18O49 composite is the best among the four CuO/WO3-x sensors. The two designs for low and high Cu/W molar ratios all achieve enhanced room-temperature H2S gas response, with a fast recovery time of ∼60 s under heating pulse, as well as an excellent selectivity, which makes the sensors a promising candidate for practical applications. Moreover, the micro-Raman spectra confirmed CuS formation and the thermal effect on the decomposition of CuS in the sensing process was studied.

Leaky electronic states for photovoltaic photodetectors based on asymmetric superlattices

The concept of leaky electronic states in the continuum is used to achieve room temperature operation of photovoltaic superlattice infrared photodetectors. A structural asymmetric InGaAs/InAlAs potential profile is designed to create states in the continuum with the preferential direction for electron extraction and, consequently, to obtain photovoltaic operation at room temperature. Due to the photovoltaic operation and virtual increase in the bandoffset, the device presents both low dark current and low noise. The Johnson noise limited specific detectivity reaches values as high as 1.4 × 1011 Jones at 80 K. At 300 K, the detectivity obtained is 7.0 × 105 Jones.

Carrier leakage into the continuum in diagonal GaAs/Al0.15GaAs terahertz quantum cascade lasers

The maximum operating temperature reported so far for THz-QCLs is ∼200 K. With the well-known degradation mechanism of thermally activated LO-phonon scattering, one straightforward strategy to improve their temperature performances is the use of diagonal structures in which the upper-to-lower state scattering time is lengthened. However, the effectiveness of this method for achieving room temperature operation remains to be demonstrated. Here, we studied the temperature degradation of highly diagonal GaAs/Al0.15GaAs THz-QCLs. By analyzing their output power dependence on temperature, we identified the physical mechanism that limits their performance to be thermally activated leakage into the continuum, as evidenced by the large activation energy of ∼80 meV extracted from the Arrhenius plot. This observation is further supported by a careful analysis of current-voltage characteristics, especially in regions of high biases. In order to significantly improve the temperature performances of diagonal THz-QCLs, this leakage should be eliminated.

Direct Au–Au bonding technology for high performance GaAs/AlGaAs quantum cascade lasers

In this paper we investigate chip bonding technology of GaAs/AlGaAs quantum cascade lasers (QCLs). Its results have strong influence on final performance of devices and are essential for achieving room temperature operation. Various solders were investigated and compared in terms of their thermal resistance and induced stress. The spatially resolved photoluminescence technique has been applied for a device thermal analysis. The soldering quality was also investigated by means of a scanning acoustic microscopy. The particular attention has been paid to Au–Au die bonding, which seems to be a promising alternative to the choice between hard and soft solder bonding of GaAs/AlGaAs QCLs operating from cryogenic temperatures up to room temperatures. A good quality direct Au–Au bonding was achieved for bonding parameters comparable with the ones typical for AuSn eutectic bonding process. High performance room temperature operation of GaAs/AlGaAs QCLs has been achieved with the state-of-the-art parameters.

Gas Sensing Behaviors of Combinatorial Structures of Graphene Oxide and ZnO

Nanoscale materials are outstanding candidates for gas-sensing elements due to extremely high surface-to-volume ratio [1]. Their unique and attractive properties can lead to novel sensors that have advantages of fast reaction speed and sensitivity, ease of miniaturization, minimum power consumption, and low cost. Recently, the expansion of gas sensor applications to medical diagnosis, wearable electronics would demand operation at room temperature and convenient fabrication of the sensors on various substrates [2].Nanostructured metal oxides such as tin oxide, zinc oxide, and graphene oxide have been demonstrated as gas sensing materials with their excellent semiconducting properties. However, the sensitivity and selectivity of the metal oxide-based gas sensors may not be sufficient for a wide range of applications. To overcome the limitations, combined graphene oxide and ZnO based sensor has been attempted by making ZnO as catalyst nanoparticles [3,4]. Such hybrid structure may present potential benefits, but the methods to construct combined structure of graphene oxide and ZnO are rarely found in the literature. Therefore, we investigate the design and fabrication of the combinatorial structure from two nanomaterials of graphene oxide and ZnO.Combinatorial structures of graphene oxide and ZnO have been performed via chemical and physical deposition methods to form layer by layer structure. Hierarchical oxide nanostructures consisting of graphene oxide flakes and ZnO nanorods were also fabricated by using electrochemical deposition. Gas sensing performance of individual graphene oxides and ZnO nanorods was shown in Fig. 1. Graphene oxide can sense various gases at room temperature, but ZnO nanorods can perform at room temperature with an aid of external energy such as UV. Thus chemical modification of ZnO nanorods was performed to achieve room temperature operation. It was also found that sensing performance of graphene oxide was strongly dependent on surface conditions or reduction of graphene oxide. Thermal reduction of deposited graphene oxide resulted in the change of reduction degree. The increase of thermal reduction temperature enabled transformation from graphene oxide (GO) to reduced graphene oxide (r-GO) as confirmed by FTIR and Raman in Fig.2. Such change also showed n-p transition of sensing behaviors. This study will present systematic investigation of nanostructured graphene oxide and ZnO for combinatorial structure. The strategies to construct combinatorial structures and their sensing behaviors in conjunction with sensing mechanisms are discussed in detail.This research was supported by Agency for Defense Development (ADD) as global corporative research for direct urine fuel cell and a grant from a Strategic Research Project (2013-0132) funded by the Korea Institute of Construction Technology.[1] S. Basu and P. Bhattacharyya, Sensors and Actuators B: Chemical(2012).[2] Q. Zhang, P. Wang, J. Li and X. Gao, Biosensors and Bioelectronics, 15, 249 (2000).[3] G. Singh, A. Choudhary, D. Haranath, A. G. Joshi, N. Singh, S. Singh and R. Pasricha, Carbon, 50, 385 (2012).[4] T. V. Cuong, V. H. Pham, J. S. Chung, E. W. Shin, D. H. Yoo, S. H. Hahn, J. S. Huh, G. H. Rue, E. J. Kim and S. H. Hur, Materials Letters, 64, 2479 (2010).

Spin split-off transition based IR detectors operating at high temperatures

GaAs/AlGaAs based Heterojunction Interfacial Workfunction Internal Photoemission (HEIWIP) detectors were used to demonstrate experimental split-off response that is based on hole transitions between light/heavy hole bands and the split-off band (spin-orbit). Preliminary results indicate that, this detection mechanism is more efficient than free carrier mechanism for NIR operation. An unoptimized, GaAs/AlGaAs detector with a free carrier threshold wavelength of ∼20μm showed a maximum operating temperature of 130K for split-off response in the range 1.5–5μm with a peak D∗ of 1.0×108Jones. By adjusting the free carrier threshold to match the split-off threshold, it should be feasible to further increase the operating temperature. Analysis indicates that practical devices with properly optimized parameters are capable of achieving room temperature operation with higher specific detectivity. The possible ways to tailor the threshold, for the split-off response to different wavelength rangers using different materials such as phosphides and nitrites are also discussed.

High operating temperature split-off band infrared detectors

Heterojunction interfacial work function internal photoemission detectors were used to demonstrate infrared response originating from hole transitions between light/heavy hole bands and the split-off (spin-orbit) band. A GaAs∕AlGaAs heterojunction with a threshold wavelength of ∼20μm indicated an operating temperature of 130K for split-off response in the range of 1.5–5μm with a peak D* of 1.0×108 Jones. Analysis suggests that practical devices with optimized parameters are capable of achieving room temperature operation with higher specific detectivity. Possible approaches to tailor the threshold for the split-off response to different wavelength ranges using different materials such as phosphides and nitrides are also discussed.

Intervalence band absorption in bulk InSb lasers

Four-band Kane theory with the effects of remote bands included via perturbation theory has been used to predict the gain spectra in the active region of bulk InSb under injection. The effect of intervalence band absorption (IVA) between light and heavy hole bands has been included by calculation from first principles and is found to increase in significance with increasing temperature. Comparison with experimental absorption spectra for doped samples shows good agreement indicating the model's validity. The authors show that to achieve room temperature operation, carrier densities of the order of 5 × 1018 cm-3 are required to overcome IVA. Such carrier densities are unlikely to be attainable in bulk devices owing to the large Auger coefficients associated with InSb. Consequently, uncooled laser operation in a device relying on purely bulk InSb properties does not seem practical.