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This paper develops a degradation test scheme for insulated gate bipolar transistors (IGBTs) that are subjected to repeated electrostatic discharge (ESD). The proposed 7-step scheme is implemented to characterize the latent ESD damage of trench gate field stop IGBTs. An empirical degradation model based on the shift in threshold voltage is proposed with three model parameters: two parameters are IGBT-type-dependent and the other parameter is manufacturer-dependent. The two IGBT-type-dependent parameters for trench gate field stop IGBTs are determined using the test results obtained from two types of IGBT components produced by the same manufacturer. The correlation between the model and the test data is extremely high, which proves the accuracy of the proposed model. The applicability of the model is further corroborated using the test data obtained from IGBT components and modules produced by other manufacturers. The results confirm that the proposed degradation model can accurately predict the degradation of IGBT components as well as modules subjected to ESD damage.

Elaborately and serially pH-modulated hydrogels possessing optimized viscoelastic natures for short gelation time and single syringe injection were designed for peritumoral injection of an anticancer agent. Boronate ester bonds between phenylboronic acid (PBA) (installed in HA-PBA (HP)) and dopamine (included in HA-dopamine (HD)) along with self-polymerization of dopamine (via interactions between HD conjugates) were introduced as the main cross-linking strategies of a hyaluronic acid (HA) hydrogel. Considering pKa values (8.0-9.5) of PBA and dopamine, the pH of each polymer dispersion was controlled elaborately for injection through a single syringe, and the final pH was tuned nearby the physiological pH (pH 7.8). The shear-thinning behavior, self-healing property, and single syringe injectability of a designed hydrogel cross-linked nearby physiological pH may provide its convenient application to peritumoral injection and prolonged retention in local cancer therapy. Erlotinib (ERT) was encapsulated in a microsphere (MS), and it was further embedded in an HP/HD-based hydrogel for sustained and locoregional delivery. A rheologically tuned hydrogel containing an ERT MS exhibited superior tumor-suppressive efficiencies compared to the other groups in A549 tumor-bearing mice. A designed injectable hydrogel through a single syringe system may be efficiently applied to local cancer therapy with lower toxicities to healthy organs.

Injectable shear-thinning biomaterials (iSTBs) have great potential for in situ tissue regeneration through minimally invasive therapeutics. Previously, an iSTB was developed by combining gelatin with synthetic silicate nanoplatelets (SNPs) for potential application to hemostasis and endovascular embolization. Hence, iSTBs are synthesized by varying compositions of gelatin and SNPs to navigate their material, mechanical, rheological, and bioactive properties. All compositions (each component percentage; 1.5-4.5%/total solid ranges; 3-9%) tested are injectable through both 5 Fr general catheter and 2.4 Fr microcatheter by manual pressure. In the results, an increase in gelatin contents causes decrease in swellability, increase in freeze-dried hydrogel scaffold porosity, increase in degradability and injection force during iSTB fabrication. Meanwhile, the amount of SNPs in composite hydrogels is mainly required to decrease degradability and increase shear thinning properties of iSTB. Finally, in vitro and in vivo biocompatibility tests show that the 1.5-4.5% range gelatin-SNP iSTBs are not toxic to the cells and animals. All results demonstrate that the iSTB can be modulated with specific properties for unmet clinical needs. Understanding of mechanical and biological consequences of the changing gelatin-SNP ratios through this study will shed light on the biomedical applications of iSTB on specific diseases.

Multicomponent biomaterials using a high-throughput system regulate hyaline or hypertrophic chondrogenesis of hMSCs., Despite great progress in biomaterial design strategies for replacing damaged articular cartilage, prevention of stem cell-derived chondrocyte hypertrophy and resulting inferior tissue formation is still a critical challenge. Here, by using engineered biomaterials and a high-throughput system for screening of combinatorial cues in cartilage microenvironments, we demonstrate that biomaterial cross-linking density that regulates matrix degradation and stiffness—together with defined presentation of growth factors, mechanical stimulation, and arginine-glycine-aspartic acid (RGD) peptides—can guide human mesenchymal stem cell (hMSC) differentiation into articular or hypertrophic cartilage phenotypes. Faster-degrading, soft matrices promoted articular cartilage tissue formation of hMSCs by inducing their proliferation and maturation, while slower-degrading, stiff matrices promoted cells to differentiate into hypertrophic chondrocytes through Yes-associated protein (YAP)–dependent mechanotransduction. in vitro and in vivo chondrogenesis studies also suggest that down-regulation of the Wingless and INT-1 (WNT) signaling pathway is required for better quality articular cartilage-like tissue production.

Gelatin methacryloyl (GelMA) is a widely used hydrogel with skin-derived gelatin acting as the main constituent. However, GelMA has not been used in the development of wearable biosensors, which are emerging devices that enable personalized healthcare monitoring. This work highlights the potential of GelMA for wearable biosensing applications by demonstrating a fully solution-processable and transparent capacitive tactile sensor with microstructured GelMA as the core dielectric layer. A robust chemical bonding and a reliable encapsulation approach are introduced to overcome detachment and water-evaporation issues in hydrogel biosensors. The resultant GelMA tactile sensor shows a high-pressure sensitivity of 0.19 kPa(−1) and one order of magnitude lower limit of detection (0.1 Pa) compared to previous hydrogel pressure sensors owing to its excellent mechanical and electrical properties (dielectric constant). Furthermore, it shows durability up to 3000 test cycles because of tough chemical bonding, and long-term stability of 3 days due to the inclusion of an encapsulation layer, which prevents water evaporation (80% water content). Successful monitoring of various human physiological and motion signals demonstrates the potential of these GelMA tactile sensors for wearable biosensing applications.

Mesenchymal stem cells (MSCs) have been widely used for regenerative therapy. In most current clinical applications, MSCs are delivered by injection but face significant issues with cell viability and penetration into the target tissue due to a limited migration capacity. Some therapies have attempted to improve MSC stability by their encapsulation within biomaterials; however, these treatments still require an enormous number of cells to achieve therapeutic efficacy due to low efficiency. Additionally, while local injection allows for targeted delivery, injections with conventional syringes are highly invasive. Due to the challenges associated with stem cell delivery, a local and minimally invasive approach with high efficiency and improved cell viability is highly desired. In this study, we present a detachable hybrid microneedle depot (d-HMND) for cell delivery. Our system consists of an array of microneedles with an outer poly(lactic-co-glycolic) acid (PLGA) shell and an internal gelatin methacryloyl (GelMA)-MSC mixture (GMM). The GMM was characterized and optimized for cell viability and mechanical strength of the d-HMND required to penetrate mouse skin tissue was also determined. MSC viability and function within the d-HMND was characterized in vitro and the regenerative efficacy of the d-HMND was demonstrated in vivo using a mouse skin wound model.

The extraction of interstitial fluid (ISF) from skin using microneedles (MNs) has attracted growing interest in recent years due to its potential for minimally invasive diagnostics and biosensors. ISF collection by absorption into a hydrogel MN patch is a promising way that requires the materials to have outstanding swelling ability. Here, we have developed a gelatin methacryloyl (GelMA) patch with an 11 × 11 array of MNs for minimally invasive sampling of ISF. The properties of the patch can be tuned by altering the concentration of the GelMA prepolymer and the crosslinking time; patches are created with swelling ratios between 293% and 423% and compressive moduli between 3.34 MPa to 7.23 MPa. The optimized GelMA MN patch demonstrated efficient extraction of ISF. Furthermore, it efficiently and quantitatively detects glucose and vancomycin in ISF in an in vivo study. This minimally invasive approach of extracting ISF with a GelMA MN patch has the potential to complement blood sampling for the monitoring of target molecules from patients.

Cells sense and transduce the chemical and mechanical properties of their microenvironment through cell surface integrin receptors. Traction stress exerted by cells on the extracellular matrix mediates focal adhesion stabilization and regulation of the cytoskeleton for directing biological activity. Understanding how stem cells integrate biomaterials properties through focal adhesions during differentiation is important for the design of soft materials for regenerative medicine. In this paper we use micropatterned hydrogels containing fluorescent beads to explore force transmission through integrins from single mesenchymal stem cells (MSCs) during differentiation. When cultured on polyacrylamide gels, MSCs will express markers associated with osteogenesis and myogenesis in a stiffness dependent manner. The shape of single cells and the composition of tethered matrix protein both influence the magnitude of traction stress applied and the resultant differentiation outcome. We show how geometry guides the spatial positioning of focal adhesions to maximize interaction with the matrix, and uncover a relationship between αvβ3, α5β1 and mechanochemical regulation of osteogenesis.

Poly(ethylene glycol) hydrogels with disulfide linkages are functionalized through applied force. Compression or tension induces bond rupture at the relatively weak disulfide linkages, which will subsequently react through Michael-type addition with an acceptor molecule within the gel. We demonstrate the utility of this approach by patterning cell adhesion proteins through compression of a lithographically structured stamp, where cells predominately adhere to the compressed regions.

A practical strategy for the reduction of nanogap width has been investigated using liquid nitrogen freezing of Pd-sputtered elastomeric substrates. Two types of hydrogen gas sensors, in which no pre-strain and 25% elongation were applied to Pd films on the elastomeric substrates, showed extremely low detection limits of less than 300 and 200 ppm, respectively, after liquid nitrogen freezing and recovery to room temperature. For the non-strained sensors, the nanogap width was measured to be about 90 nm, whereas it was more reduced to 25 nm on the elongated sensors. Both sensors exhibited perfect On–Off switching, fast response, and good reversibility, which are based on the nanogap open–close mechanism upon exposure to hydrogen gas. These results provide a valuable clue for reducing nanogap width, thereby improving the hydrogen-sensing capabilities of nanogap-based On–Off hydrogen sensors.

Cell activity is coordinated by dynamic interactions with the extracellular matrix, often through stimuli-mediated spatiotemporal stiffening and softening. Dynamic changes in mechanics occur in vivo through enzymatic or chemical means, processes which are challenging to reconstruct in cell culture materials. Here a magnetoactive hydrogel material formed by embedding magnetic particles in a hydrogel matrix is presented whereby elasticity can be modulated reversibly by attenuation of a magnetic field. Orders of magnitude change in elasticity using low magnetic fields are shown and reversibility of stiffening with simple permanent magnets is demonstrated. The broad applicability of this technique is demonstrated with two therapeutically relevant bioactivities in mesenchymal stem cells: secretion of proangiogenic molecules, and dynamic control of osteogenesis. The ability to reversibly stiffen cell culture materials across the full spectrum of soft tissue mechanics, using simple materials and commercially available permanent magnets, makes this approach viable for a broad range of laboratory environments.

We have successfully fabricated sub-100 nm nanogaps in PdeNi alloy thin films on an elastomeric substrate by simple stretching. The nanogaps-containing PdeNi films were utilized as hydrogen-sensing sponges and their performance was demonstrated to dominate over the performance of similar mobile thin films comprised of pure Pd in major aspects such as the response time, sensitivity in high H2 concentrations, and H2 detection limit. Notably, Pd87.5Ni12.5 hydrogen sensing sponges showed ultra-high sensitive and reversible On-Off behaviors and low detection limit of w100 ppm, which were attributed to the reduced nanogap width and the enhanced volume expandability of PdeNi lattice. The effects of Ni added to Pd and a search for an optimum Ni concentration were also systematically studied.

We have investigated a lithography-free technique for On-Off type hydrogen sensors using a cracked palladium (Pd) film on an elastomeric substrate. Cracks were induced in a sputtered Pd film simply by undergoing hydrogen absorption and desorption processes. Compared to the same thickness of a Pd film on a Si/SiO 2 substrate that relied on the electron scattering mechanism, a cracked Pd film on an elastomeric substrate operated as a reversible On-Off hydrogen sensor based on the crack open-close mechanism when exposed to hydrogen. The thickness of a Pd film on the elastomeric substrate plays a significant role in determining the sensing mode of the cracked Pd film. The cracked Pd film with a thickness of 9–11 nm on the elastomeric substrate showed reversible and perfect On-Off responses under a wide range of hydrogen concentrations with large current variations and a fast response time of less than 1 s.

We report the fabrication of a novel gas sensor that utilizes electrical resistance changes in electrically conductive polyaniline (PANI) and single-walled carbon nanotube (SWNT) composite materials and its sensing property when NH3 and CO gases co-existed. In addition, we investigated the concentration dependence of electrical properties of the PANI/SWNT composite material at room temperature using real-time monitoring. To improve the gas sensor properties, we deposited PANI using a drop-casting method to warp the PANI surrounding the SWNT. The PANI/SWNT composite material sensors showed a faster response to NH3 gas than CO gas. The CO gas increased the composite conductance, while the NH3 gas had the opposite effect. Open image in new window

We report on the effects of surface roughness resulting from an ion milling technique on the hydrogen gas sensing performance of a single Pd nanowire grown by electrodeposition into nanochannels in anodized aluminum oxide templates. A combination of electron beam lithography and a lift-off process was utilized to fabricate four-terminal devices based on individual Pd nanowires. These results are the first demonstration of the effect of ion milling on the response time in a single Pd nanowire used as a hydrogen sensor. The response time of the single Pd nanowire surface-treated by ion milling was 20 times faster than that of a sample without surface treatment. The faster response time was due to the surface roughness effects of the surface treatment, an increase in the surface-to-volume ratio of the ion-milled nanowire.

Advances in biomaterial synthesis and fabrication, stem cell biology, bioimaging, microsurgery procedures, and microscale technologies have made minimally invasive therapeutics a viable tool in regenerative medicine. Therapeutics, herein defined as cells, biomaterials, biomolecules, and their combinations, can be delivered in a minimally invasive way to regenerate different tissues in the body, such as bone, cartilage, pancreas, cardiac, skeletal muscle, liver, skin, and neural tissues. Sophisticated methods of tracking, sensing, and stimulation of therapeutics in vivo using nano-biomaterials and soft bioelectronic devices provide great opportunities to further develop minimally invasive and regenerative therapeutics (MIRET). In general, minimally invasive delivery methods offer high yield with low risk of complications and reduced costs compared to conventional delivery methods. Here, minimally invasive approaches for delivering regenerative therapeutics into the body are reviewed. The use of MIRET to treat different tissues and organs is described. Although some clinical trials have been performed using MIRET, it is hoped that such therapeutics find wider applications to treat patients. Finally, some future perspective and challenges for this emerging field are highlighted.

We investigated the hydrogen (H2) sensing properties of palladium (Pd)–nickel (Ni) alloy films with varying Ni content and discussed them in light of structural deformations. The Pd–Ni alloys operated reversibly upon H2 absorption and desorption and their sensitivities decreased linearly with Ni content added to Pd. This was attributed to reduction in the lattice constant and interstitial volume caused by the Ni addition, allowing fewer hydrogen atoms to penetrate into the Pd–Ni alloy with higher Ni content. Interestingly, the response time of the Pd–Ni alloys was much shorter than that of pure Pd, presumably due to the fast permeation of hydrogen atoms through microscopic imperfections in the alloys. Unlike pure Pd, the Pd–Ni alloys showed an almost linear relationship between the sensitivity and H2 concentration without hysteretic behaviors, enabling the detection of low concentration of H2 down to 0.01%. These results provide a significant understanding of the role of Ni in the Pd–Ni thin films for improving the H2 sensing properties of the Pd-based alloy film sensors.

We have investigated the fabrication of hydrogen gas sensors based on networks of Pd nanoparticles (NPs) deposited tin dioxide nanowires (NWs). SnO2 NWs with tin NPs attached on the surface were obtained by a simple thermal evaporation of SnO crystalline powders. The tin dioxide NWs were decorated with Pd NPs by the reduction process in Pd ion solution. The sensors showed ultra-high sensitivity (∼1.2 × 105%) and fast response time (∼2 s) upon exposure to 10,000 ppm H2 at room temperature. These sensors were also found to enable a significant electrical conductance modulation upon exposure to extremely low concentrations (40 ppm) of H2 in the air. Our fabrication method of sensors combining with Pd NPs, Sn NPs and n-type semiconducting SnO2 NWs allows optimized catalytic and depletion effect and results the production of highly-sensitive H2 sensors that exhibit a broad dynamic detection range, fast response times, and an ultra-low detection limit.

We investigated the effects of a thin titanium (Ti) buffer layer on structural changes and electrical responses of palladium (Pd) thin films. A Ti buffer layer was inserted between a Pd film and the substrate, with varying thickness from 0.5 to 80 nm. Unlike pure Pd films, Ti-buffered Pd films showed no structural deformations after cyclic exposure to hydrogen gas, leading to a linear relationship between sensitivity and hydrogen gas concentration over the measured concentration range of 0 to 2%. This was attributed to the suppression of phase transitions from the α to the β phase in Pd films, due to the reinforced film adhesion by the inserted Ti layer. Our results highlight the practical usability of Pd thin films as reliable and sensible hydrogen sensors, enabled simply by the insertion of a thin Ti buffer layer.

We report the fabrication of a novel hydrogen sensor that utilizes the electrical resistance changes in the palladium thin films with nanometer thicknesses. The sensing mechanism is based on transitory absorption of hydrogen atoms into the palladium layer, which leads to the reversible alteration of the electrical resistance. In concentrated hydrogen ambient, the excess hydrogen absorption process leads to mechanical deformation on the surface of the palladium films, corresponding to the phase transition from α-phase to β-phase. The reversible sensing process results in a hysteresis curve for resistive properties, of which the height (sensitivity) could be controlled by manipulating the thickness of the palladium layers. The peel-off phenomena on the surface of the palladium film were suppressed by decreasing the thickness of the film. At the thickness of 20 nm, a hysteresis curve of resistance was obtained without any structural change in the palladium thin film. These results provide a significant insight to the fundamental understanding of the relationship between the electrical sensitivity of pure Pd thin films and related structural deformation, which is essential to develop robust H-sensors with high sensibility.

We have investigated the hydrogen gas sensing performance of single-walled carbon nanotubes (SWCNTs) grafted with Pd nanoparticles (NPs). The SWCNTs, modified with dendrimers before grafting, were found to have a much faster response time (3 s) and better recovery but lower response (8.6%) at 10,000 ppm hydrogen gas than those prepared without the dendrimer template (253 s, 25%). This can be ascribed to the dipole moments induced by the dendrimers. Removal of the dendrimers by heat treatment resulted in high response (25%), a fast response time (7 s) at 10,000 ppm hydrogen gas, and an ultra-low concentration H2 detection of 10 ppm at room temperature. In addition, this sample was found to satisfy Sievert’s law. Our results demonstrate that SWCNTs grafted with Pd-NPs through a dendrimermediated process can be used to fabricate highly sensitive hydrogen gas sensors that exhibit a broad dynamic detection range, fast response times, and excellent recovery.

The aim of the present study was to investigate surface characteristics in four different titanium surfaces (AN: anodized at 270 V; AN-CaP: anodic oxidation and CaP deposited; SLA: sandblasted and acid etched; MA: machined) and to evaluate biological behaviors such as cell adhesion, cell proliferation, cytoskeletal organization, and osteogenic protein expression of MG63 osteoblast-like cells at the early stage. Surface analysis was performed using scanning electron microscopy, thin-film X-ray diffractometry, and a confocal laser scanning microscope. In order to evaluate cellular responses, MG63 osteoblast-like cells were used. The cell viability was evaluated by MTT assay. Immunofluorescent analyses of actin, type I collagen, osteonectin and osteocalcin were performed. The anodized and CaP deposited specimen showed homogeneously distributed CaP particles around micropores and exhibited anatase type oxides, titanium, and HA crystalline structures. This experiment suggests that CaP particles on the anodic oxidation surface affect cellular attachment and spreading. When designing an in vitro biological study for CaP coated titanium, it must be taken into account that preincubation in medium prior to cell seeding and the cell culture medium may affect the CaP coatings. All these observations illustrate the importance of the experimental conditions and the physicochemical parameters of the CaP coating. It is considered that further evaluations such as long-term in vitro cellular assays and in vivo experiments should be necessary to figure out the effect of CaP deposition to biological responses.

The reliable generation of smooth muscle cells is important for a number of tissue engineering applications. Human mesenchymal stem cells (MSCs) are a promising progenitor of smooth muscle, with high expression of smooth muscle markers observed in a fraction of isolated cells, which can be increased by introduction of soluble supplements that direct differentiation. Here we demonstrate a new micropatterning technique, where peptides of different ligand affinity can be microcontact printed onto an inert background, to explore MSC differentiation to smooth muscle through controlled biochemical and biophysical cues alone. Using copper-catalyzed alkyne-azide cycloaddition (CuAAC), we patterned our surfaces with RGD peptide ligands-both a linear peptide with low integrin affinity and a cyclic version with high integrin affinity-for the culture of MSCs in shapes with various aspect ratios. At low aspect ratio, ligand affinity is a prime determinant for smooth muscle differentiation, while at high aspect ratio, ligand affinity has less of an effect. Pathway analysis reveals a role for focal adhesion turnover, Rac1, RhoA/ROCK, and calpain during smooth muscle differentiation of MSCs in response to cell shape and the affinity of the cell adhesion interface. Controlling integrin-ligand affinity at the biomaterials interface is important for mediating adhesion but may also prove useful for directing smooth muscle myogenesis. Peptide patterning enables the systematic investigation of single to multiple peptides derived from any protein, at different densities across a biomaterials surface, which has the potential to direct multiple MSC differentiation outcomes without the need for soluble supplements.

An AlGaN/GaN high-electron-mobility transistor (HEMT) with a novel source-connected air-bridge field plate (AFP) is proposed. The device features a metal field plate (FP) that jumps from the source over the gate region and lands between the gate and drain. The fabrication process is based on a commercially available radio-frequency GaN-on-SiC technology. Device characteristics for this work were optimized via layout changes only. When compared to a similar-sized HEMT device with a conventional FP structure, the AFP not only minimizes the parasitic gate-to-source capacitance but also exhibits a higher off-state breakdown voltage and one order of magnitude lower drain leakage current. In a device with a gate-to-drain distance of 6 μm and a gate length of 0.8 μm, a three-times-higher forward blocking voltage of 375 V was obtained at VGS = -5 V. In contrast, a similar-sized HEMT with a conventional FP can only achieve a breakdown voltage no higher than 125 V using this process, regardless of device dimensions. The specific on-resistance for the device with the proposed AFP is 0.58 mΩ·cm2 at VGS = 0 V, which compares favorably with 0.79 mΩ·cm2 for the best device with a conventional FP.

The physical properties of the extracellular matrix (ECM) play an important role in regulating tissue-specific human mesenchymal stem cell (MSC) differentiation. Protein-coated hydrogels with tunable stiffness have been shown to influence lineage specific gene expression in MSCs. In addition, the control of cell shape - either through changing substrate stiffness or restricting spreading with micropatterning - has proved to be important in guiding the differentiation of MSCs. However, few studies have explored the interplay between these physical cues during MSC lineage specification. Here, we demonstrate geometric control of osteogenesis in MSCs cultured on micropatterned polyacrylamide gels. Cells cultured on fibronectin-coated gels express markers associated with osteogenesis in a stiffness dependent fashion with a maximum at ~30kPa. Controlling the geometry of single cells across the substrate demonstrates elevated osteogenesis when cells are confined to shapes that promote increased cytoskeletal tension. Patterning MSCs across hydrogels of variable stiffness will enable the exploration of the interplay between these physical cues and their relationship with the mechanochemical signals that guide stem cell fate decisions.

We report on a novel method for the fabrication of highly sensitive hydrogen gas sensors based on palladium oxide thin films and have investigated their hydrogen sensing properties and nanostructures. To our knowledge, this is the first report on the use of palladium oxide and reduced palladium thin films as hydrogen sensors. The palladium oxide thin films were deposited on thermally oxidized Si substrates using a reactive direct current (DC) magnetron sputtering system. Considerable changes in the resistance of the palladium oxide thin films were observed when they were initially exposed to hydrogen gas, as a result of the reduction process. After the initial exposure to hydrogen gas of PdO(30%), its sensitivity increased up to approximately 4.5 x 10(3)%. The morphology of the PdO surface was analyzed using a scanning electron microscope (SEM), in order to investigate the interactions between palladium oxide and hydrogen. The SEM images showed a large number of nano-sized cracks on the surface of the palladium oxide during the reduction process, which acted to increase the effective surface-to-volume ratio. The response behaviors of the reduced Pd films to hydrogen gas were reversible and had an enhanced sensing property when compared with those of the pure Pd films. In addition, their sensitivities and response times were improved due to the nano-sized cracks on the surfaces. The results demonstrate that palladium oxide and reduced palladium thin films can be applied for use in highly sensitive hydrogen sensors.

We report the electrical detection of nerve agent gas using p-type Si-nanowire-based sensors. A combination of electron-beam lithography and a lift-off process was utilized to fabricate individual 50-nm-thick Si nanowire sensors. Using the fabricated Si-nanowire sensors, the detection of DMMP was successfully achieved. Our results suggest that Si-nanowire-based gas sensors can be used to detect dimethylniethylphosphonate (DMMP).

We report on the hydrogen gas (H2) sensing performance of lithographically patterned Pd nanowires as a function of the nanowire thickness and H2 concentration. A combination of electron-beam lithography and a lift-off process has been utilized to fabricate four-terminal devices based on individual Pd nanowires with width w = 300 nm, length l = 10 microm, and thickness t = 20-400 nm from continuous Pd films. The variation of the resistance and sensitivity at 20 000 ppm H2 of Pd nanowires was found to be much lager than at 10 000 ppm H2, which can be explained by an alpha-beta phase transition occurring at 20 000 ppm H2. This is confirmed by the observation of hysteresis behavior in the resistance versus H2 concentration for Pd thin films. The response time was found to decrease with decreasing thickness regardless of H2 concentration due to a higher surface-to-volume ratio and a higher clamping effect. A single Pd nanowire with t = 100 nm was found to successfully detect H2 at a detection limit of 20 ppm. Our results suggest that lithographically patterned Pd nanowires can be used as hydrogen gas sensors to quantitatively detect H2 over a wide range of concentrations.