Pre-patterned Electrodes Research Articles

High-detectivity photodetectors based on CsPbCl3/Au heterostructures

Photodetector based on all-inorganic CsPbCl3 perovskite is drawing increasing interest owing to its stable chemical properties, suitable band gap and superior inherent optoelectronic properties. However, most of the perovskite detectors are fabricated from polycrystalline films with a number of grain boundaries and defects which would decline the device performance. At the same time, the performance of the device is sacrificed by the presence of a large amount of gaps between the metal electrodes and the material. Here, a high detective photodetector is demonstrated based on CsPbCl3 crystals by liquid-phase growth on pre-patterned Au electrodes. Heterojunctions fabricated by liquid-phase self-assembly synthesis form an embedded structure between metal electrodes and the material, which greatly enhances the contact area and reduces the dark current. The device serves as photodetectors with a responsivity of 62.5 μA/W and detectivity up to 1.79 × 1011 Jones. Furthermore, these devices exhibit outstanding performance of field-effect transistors with a remarkable on/off ratio of 107. These results offer novel insights for the exploration of high-performance photodetectors based on all-inorganic perovskite materials.

Single-crystalline YIG flakes with uniaxial in-plane anisotropy and diverse crystallographic orientations

We study sub-micron Y3Fe5O12 (YIG) flakes that we produce via mechanical cleaving and exfoliation of YIG single crystals. By characterizing their structural and magnetic properties, we find that these YIG flakes have surfaces oriented along unusual crystallographic axes and uniaxial in-plane magnetic anisotropy due to their shape, both of which are not commonly available in YIG thin films. These physical properties, combined with the possibility of picking up the YIG flakes and stacking them onto flakes of other van der Waals materials or pre-patterned electrodes or waveguides, open unexplored possibilities for magnonics and for the realization of novel YIG-based heterostructures and spintronic devices.

Strain-restricted transfer of ferromagnetic electrodes for constructing reproducibly superior-quality spintronic devices

Spintronic device is the fundamental platform for spin-related academic and practical studies. However, conventional techniques with energetic deposition or boorish transfer of ferromagnetic metal inevitably introduce uncontrollable damage and undesired contamination in various spin-transport-channel materials, leading to partially attenuated and widely distributed spintronic device performances. These issues will eventually confuse the conclusions of academic studies and limit the practical applications of spintronics. Here we propose a polymer-assistant strain-restricted transfer technique that allows perfectly transferring the pre-patterned ferromagnetic electrodes onto channel materials without any damage and change on the properties of magnetism, interface, and channel. This technique is found productive for pursuing superior-quality spintronic devices with high controllability and reproducibility. It can also apply to various-kind (organic, inorganic, organic-inorganic hybrid, or carbon-based) and diverse-morphology (smooth, rough, even discontinuous) channel materials. This technique can be very useful for reliable device construction and will facilitate the technological transition of spintronic study.

Catalytic synergy of WS2-anchored PdSe2 for highly sensitive hydrogen gas sensor.

Hydrogen (H2) is widely used in industrial processes and is one of the well-known choices for storage of renewable energy. H2 detection has become crucial for safety in manufacturing, storage, and transportation due to its strong explosivity. To overcome the issue of explosion, there is a need for highly selective and sensitive H2 sensors that can function at low temperatures. In this research, we have adequately fabricated an unreported van der Waals (vdWs) PdSe2/WS2 heterostructure, which exhibits exceptional properties as a H2 sensor. The formation of these heterostructure devices involves the direct selenization process using chemical vapor deposition (CVD) of Pd films that have been deposited on the substrate of SiO2/Si by DC sputtering, followed by drop casting of WS2 nanoparticles prepared by a hydrothermal method onto device substrates including pre-patterned electrodes. The confirmation of the heterostructure has been done through the utilization of powder X-ray diffraction (XRD), depth-dependent X-ray photoelectron spectroscopy (XPS) and field-emission scanning electron microscopy (FE-SEM) techniques. Also, the average roughness of thin films is decided by Atomic Force Microscopy (AFM). The comprehensive research shows that the PdSe2/WS2 heterostructure-based sensor produces a response that is equivalent to 67.4% towards 50 ppm H2 at 100 °C. The response could be a result of the heterostructure effect and the superior selectivity for H2 gas in contrast to other gases, including NO2, CH4, CO and CO2, suggesting tremendous potential for H2 detection. Significantly, the sensor exhibits fast response and a recovery time of 31.5 s and 136.6 s, respectively. Moreover, the explanation of the improvement in gas sensitivity was suggested by exploiting the energy band positioning of the PdSe2/WS2 heterostructure, along with a detailed study of variations in the surface potential. This study has the potential to provide a road map for the advancement of gas sensors utilizing two-dimensional (2D) vdWs heterostructures, which exhibit superior performance at low temperatures.

Individually-addressable composite microneedle electrode array by mold-and-place method for glucose detection

Microneedle (MN)-based electrochemical biosensors enable minimally invasive and in situ monitoring of biomolecules from interstitial fluid (ISF) that is strongly correlated to their concentrations in the blood. When an array of MNs is used as either working or counter electrodes for electrochemical sensing, it is often difficult to have narrow spacing between them and this can lead to poor measurement accuracy due to increased resistance, low sensitivity, and slow response time. This study aims to develop a method to fabricate independently functioning MN electrodes with narrow intervals between them for high precision electrochemical sensing. A mixture of photocurable polymer (SU-8) and single-wall carbon nanotubes (SWCNTs) was optimized for pressure-assisted transfer (PAT) molding and electrical conductivity. Single composite MNs were molded by PAT molding, and then attached on pre-patterned electrodes. After the ‘mold-and-place’ of the composite MN structures, plasma etching and electropolymerization of PEDOT:PSS were performed to enhance the electrochemical activity. Prussian blue (PB) and glucose oxidase (GOx) were electrodeposited on surface-treated MNs to enable glucose detection. MN electrodes showed limit of detection (LOD) at 0.225 mM and linearity up to 20 mM. Finally, MN electrodes were constructed on a flexible polyimide film and demonstrated the feasibility of detecting glucose in an in vivo mouse study.

MoRe Electrodes with 10 nm Nanogaps for Electrical Contact to Atomically Precise Graphene Nanoribbons.

Atomically precise graphene nanoribbons (GNRs) are predicted to exhibit exceptional edge-related properties, such as localized edge states, spin polarization, and half-metallicity. However, the absence of low-resistance nanoscale electrical contacts to the GNRs hinders harnessing their properties in field-effect transistors. In this paper, we make electrical contact with nine-atom-wide armchair GNRs using superconducting alloy MoRe as well as Pd (as a reference), which are two of the metals providing low-resistance contacts to carbon nanotubes. We take a step toward contacting a single GNR by fabricating electrodes with needlelike geometry, with about 20 nm tip diameter and 10 nm separation. To preserve the nanoscale geometry of the contacts, we develop a PMMA-assisted technique to transfer the GNRs onto the prepatterned electrodes. Our device characterizations as a function of bias voltage and temperature show thermally activated gate-tunable conductance in GNR-MoRe-based transistors.

One-step Spray coating CsPbBr3 film for planar photodetector applications

Metal halide perovskite, a promising semiconductor material, is suitable for new generation optoelectronic application. In comparison to organic -inorganic hybrid perovskites (OIHP), all inorganic CsPbX3 (X = Cl, Br and I) show superior thermal and chemical stability. Although there are many kinds of coating procedures for producing perovskite QDs peorovskite film, most studies of CsPbX3 perovskite thin film fabrication focus on solution processing techniques such as spin coating. Unfortunately, the CsPbBr3 precusor dissolubility is very low compared to OIHP, resulting in a low materials utilization and poor uniformity for the large-scale area. In this work, we report a facile spray coating method to produce CsPbBr3 film on pre-patterned Pt electrode for photodetector (PD) application. The spray-coated perovskite was demonstrated suitable for photodiode application with numbers figure of merit such as: A high responsivity of 60 A/W, high detectivity of 6× 10-13 Jones, the response time (rising and fall time) below 17 ms were obtained at 1 V under 405 nm illumination. From this study, we belive that the spray deposition technique will benefit for futher thin film-based perovskite optoelectronic devices.

Platinum contacts for 9-atom-wide armchair graphene nanoribbons

Creating a good contact between electrodes and graphene nanoribbons (GNRs) has been a long-standing challenge in searching for the next GNR-based nanoelectronics. This quest requires the controlled fabrication of sub-20 nm metallic gaps, a clean GNR transfer minimizing damage and organic contamination during the device fabrication, as well as work function matching to minimize the contact resistance. Here, we transfer 9-atom-wide armchair-edged GNRs (9-AGNRs) grown on Au(111)/mica substrates to pre-patterned platinum electrodes, yielding polymer-free 9-AGNR field-effect transistor devices. Our devices have a resistance in the range of 106–108 Ω in the low-bias regime, which is 2–4 orders of magnitude lower than previous reports. Density functional theory calculations combined with the non-equilibrium Green's function method explain the observed p-type electrical characteristics and further demonstrate that platinum gives strong coupling and higher transmission in comparison to other materials, such as graphene.

Electrical Contact Resistance of Large-Area Graphene on Pre-Patterned Cu and Au Electrodes.

Contact resistance between electrically connected parts of electronic elements can negatively affect their resulting properties and parameters. The contact resistance is influenced by the physicochemical properties of the connected elements and, in most cases, the lowest possible value is required. The issue of contact resistance is also addressed in connection with the increasingly frequently used carbon allotropes. This work aimed to determine the factors that influence contact resistance between graphene prepared by chemical vapour deposition and pre-patterned Cu and Au electrodes onto which graphene is subsequently transferred. It was found that electrode surface treatment methods affect the resistance between Cu and graphene, where contact resistance varied greatly, with an average of 1.25 ± 1.54 kΩ, whereas for the Au electrodes, the deposition techniques did not influence the resulting contact resistance, which decreased by almost two orders of magnitude compared with the Cu electrodes, to 0.03 ± 0.01 kΩ.

Comparison of electrical and optical transduction modes of DNA-wrapped SWCNT nanosensors for the reversible detection of neurotransmitters.

In this study, we compare the electrical and optical signal transduction of nanoscale biosensors based on single-walled carbon nanotubes (SWCNTs). Solution processable single-stranded (ss) DNA-wrapped SWCNTs were used for the fabrication of the distinct sensors. For electrical measurements, SWCNTs were assembled from solution onto pre-patterned electrodes by electric-field-assisted assembly in field-effect transistor (FET) configuration. A combination of micro- and nano-fabrication and microfluidics enabled the integration into a sensing platform that allowed real-time and reversible detection. For optical measurements, the near-infrared (NIR) fluorescence of the SWCNTs was acquired directly from solution. The detection of important biomolecules was investigated in high-ionic strength solution (0.5xPBS). Increase in fluorescence intensities correlated with a decrease in the SWCNTs electrical current and enabled detection of the important biomolecules dopamine, epinephrine, and ascorbic acid. For riboflavin, however, a decrease in the fluorescence intensity could not be associated with changes in the SWCNTs electrical current, which indicates a different sensing mechanism. The combination of SWCNT-based electrical and optical transduction holds great potential for selective detection of biomarkers in next generation portable diagnostic assays.

Demonstration of Molecular Tunneling Junctions Based on Vertically Stacked Graphene Heterostructures

We demonstrate the fabrication and complete characterization of vertical molecular tunneling junctions based on graphene heterostructures, which incorporate a control series of arylalkane molecules acting as charge transport barriers. Raman spectroscopy and atomic force microscopy were employed to identify the formation of the molecular monolayer via an electrophilic diazonium reaction on a pre-patterned bottom graphene electrode. The top graphene electrode was transferred to the deposited molecular layer to form a stable electrical connection without filamentary damage. Then, we showed proof of intrinsic charge carrier transport through the arylalkane molecule in the vertical tunneling junctions by carrying out multiprobe approaches combining complementary transport characterization methods, which included length- and temperature-dependent charge transport measurements and transition voltage spectroscopy. Interpretation of all the electrical characterizations was conducted on the basis of intact statistical analysis using a total of 294 fabricated devices. Our results and analysis can provide an objective criterion to validate molecular electronic devices fabricated with graphene electrodes and establish statistically representative junction properties. Since many of the experimental test beds used to examine molecular junctions have generated large variation in the measured data, such a statistical approach is advantageous to identify the meaningful parameters with the data population and describe how the results can be used to characterize the graphene-based molecular junctions.

A Facile Centrifuge Coating Method for High-Performance CsPbBr3 Compact and Crack-Free Nanocrystal Thin Film Photodetector

All-inorganic perovskite quantum dots (QDs), a promising semiconductor material, is suitable for new generation optoelectronic application. While there are many kinds of coating procedures for producing perovskite QDs peorovskite film, those methods require post-treatments and an additional dispersion support agent while still retaining pinholes and cracks. In this work, we report a facile method to produce CsPbBr3 film on a pre-patterned Pt electrode using a centrifuge coating method for photodetector (PD) application. Compact and crack-free films with ~500 nm thick from various particle sizes of 8 nm, 12 nm, and >30 nm were achieved with a suitable ratio of toluene/ethyl acetate solvent for visible light photodetector application. The optimized device has an on/off ratio of 103, detectivity of 3 × 1012 Jones, and responsivity of 6 A/W. In comparison, the on/off ratio of the device fabricated by the centrifuge coating method was 102 times higher than by the drop-coating method. The PD performance exhibited considerable moisture stability at mild high ambient temperature with no encapsulation for more than two weeks. The results suggest that this is a potential method for fabricating all inorganic perovskite nano-semiconductor films for further optoelectronic application in photodetectors, LEDs, and solar cells.

Polymer assisted transferring of patterned electrodes for a weak anisotropic charge transport study in monolayer molecular crystals

Here, by taking the advantage of the varied adhesion forces between substrates/gold electrodes/polymer films, the prepatterned fan-shaped electrodes could be readily transferred to target MMCs for the anisotropy charge transport study.

Effects of Insertion of Ag Mid-Layers on Laser Direct Ablation of Transparent Conductive ITO/Ag/ITO Multilayers: Role of Effective Absorption and Focusing of Photothermal Energy.

From the viewpoint of the device performance, the fabrication and patterning of oxide–metal–oxide (OMO) multilayers (MLs) as transparent conductive oxide electrodes with a high figure of merit have been extensively investigated for diverse optoelectronic and energy device applications, although the issues of their general concerns about possible shortcomings, such as a more complicated fabrication process with increasing cost, still remain. However, the underlying mechanism by which a thin metal mid-layer affects the overall performance of prepatterned OMO ML electrodes has not been fully elucidated. In this study, indium tin oxide (ITO)/silver (Ag)/ITO MLs are fabricated using an in-line sputtering method for different Ag thicknesses on glass substrates. Subsequently, a Q-switched diode-pumped neodymium-doped yttrium vanadate (Nd:YVO4, λ = 1064 nm) laser is employed for the direct ablation of the ITO/Ag/ITO ML films to pattern ITO/Ag/ITO ML electrodes. Analysis of the laser-patterned results indicate that the ITO/Ag/ITO ML films exhibit wider ablation widths and lower ablation thresholds than ITO single layer (SL) films. However, the dependence of Ag thickness on the laser patterning results of the ITO/Ag/ITO MLs is not observed, despite the difference in their absorption coefficients. The results show that the laser direct patterning of ITO/Ag/ITO MLs is primarily affected by rapid thermal heating, melting, and vaporization of the inserted Ag mid-layer, which has considerably higher thermal conductivity and absorption coefficients than the ITO layers. Simulation reveals the importance of the Ag mid-layer in the effective absorption and focusing of photothermal energy, thereby supporting the experimental observations. The laser-patterned ITO/Ag/ITO ML electrodes indicate a comparable optical transmittance, a higher electrical current density, and a lower resistance compared with the ITO SL electrode.

High-transconductance silicon carbide nanowire-based field-effect transistor (SiC-NWFET) for high-temperature applications

PurposeThe purpose of this study is to present a systematic investigation of the effect of high temperatures on transport characteristics of nitrogen-doped silicon carbide nanowire-based field-effect transistor (SiC-NWFET). The 3C-SiC nanowires can endure high-temperature environments due to their wide bandgap, high thermal conductivity and outstanding physical and chemical properties.Design/methodology/approachThe metal-organic chemical vapor deposition process was used to synthesize in-situ nitrogen-doped SiC nanowires on SiO2/Si substrate. To fabricate the proposed SiC-NWFET device, the dielectrophoresis method was used to integrate the grown nanowires on the surface of pre-patterned electrodes onto the SiO2 layer on a highly doped Si substrate. The transport properties of the fabricated device were evaluated at various temperatures ranging from 25°C to 350°C.FindingsThe SiC-NWFET device demonstrated an increase in conductance (from 0.43 mS to 1.2 mS) after applying a temperature of 150°C, and then a decrease in conductance (from 1.2 mS to 0.3 mS) with increasing the temperature to 350°C. The increase in conductance can be attributed to the thermionic emission and tunneling mechanisms, while the decrease can be attributed to the phonon scattering. Additionally, the device revealed high electron and hole mobilities, as well as very low resistivity values at both room temperature and high temperatures.Originality/valueHigh-temperature transport properties (above 300°C) of 3C-SiC nanowires have not been reported yet. The SiC-NWFET demonstrates a high transconductance, high electron and hole mobilities, very low resistivity, as well as good stability at high temperatures. Therefore, this study could offer solutions not only for high-power but also for low-power circuit and sensing applications in high-temperature environments (∼350°C).

Visible photodetector based on transition metal-doped ZnO NRs/PEDOT:PSS hybrid materials.

A hybrid Cu-doped ZnO nanorods (ZnO:Cu NRs)/poly(3,4 ethylene dioxythiophene)-polystyrene sulfonate (PEDOT:PSS)-based photodetector was fabricated using a simple hydrothermal method with pre-patterned silver electrodes. In the hybrid structure, PEDOT:PSS with high mobility acts as a carrier transport layer, while ZnO:Cu NRs with high visible absorption works as an “antenna” material to generate electron–hole pairs under light illumination. As a result, the devices exhibits a high response in visible light at a wavelength of 395 nm. The responsivity and photoconductive gain of the hybrid photodetector reached 0.33 A W−1 and 1.306, respectively, which is 1.36 times higher than those of Cu-doped ZnO NRs-based ones. The response and recovery times are improved, with values of 25.21 s and 42.01 s, respectively. The development of hybrid materials for visible photodetectors enables an innovative approach for future optoelectronic devices, especially optical sensors.

Facile preparation of N-S co-doped graphene quantum dots (GQDs) from graphite waste for efficient humidity sensing

In this work, graphene quantum dots (GQDs) were prepared from Graphitic waste. The resulting GQDs were evaluated for the potential application for resistive humidity sensors. The resistive humidity sensors were fabricated on the pre-patterned interdigital ITO electrodes using the three different concentrations (2.5, 5.0, and 10 mg) of GQDs in DMF. The GQDs films were deposited using the spin coating technique. The GQDs (10 mg/ml) based impedance sensors showed good sensitivity and lowered hysteresis as compared to the other ratios (2.5 and 5 mg) of the GQDs. The maximum calculated hysteresis of the GQDs (10 mg) based humidity sensor is around 2.2 % at 30%RH, and the minimum calculated hysteresis of the GQDs (10 mg/ml) based humidity sensor is approximately 0.79 % at 60 %RH. The response and recovery time found to be 15 s and 55 s, respectively. The interesting humidity-dependent resistive properties of these prepared GQDs make them promising for potential application in humidity sensing.

Perovskite Granular Wire Photodetectors with Ultrahigh Photodetectivity.

Control over the morphology and crystallinity of metal halide perovskite materials is of key importance to enable high-performance optoelectronics. Here, a simple yet effective template-free self-assembly synthesis of perovskite granular wires with ultrahigh photodetectivity (3.17 × 1015 Jones) is reported. The 1D self-assembly of perovskite grains is driven by differences in the surface interaction energies of the granular facets. The superb photodetecting performance originates from extremely low dark current engendered by energetic barriers featuring unique band-edge modulation along the long axis of wire. Flexible photodetector arrays, fabricated by selectively placing perovskite granular wires onto pre-patterned electrode arrays on a transparent polymer substrate, show independently addressable photonic signal mapping with remarkably high detectivity, photoconductive gain, and responsivity. The "self-assembled nanograin engineering" strategy developed in this study provides a viable method for the development of high-performance perovskite photodetectors and can be extended to other integrated optoelectronic systems.

Microscale surface potential gradient disturbances observed in bilayer graphene

Van der Waals mono- and heterostructures have received great attention in recent years due to their new physics and electronic properties. Several novel device structures have been developed based on their properties, including tunnelling transistors, light emitting diodes and barristors. However, the construction of such structures usually involves transfer techniques which could induce damage and modify the electronic properties of the structures and degrade device performance. Here we show that a multi-step transfer process for graphene may disturb its transport properties at microscale. Bilayer graphene was transferred, using PMMA as supporting material, onto an Al2O3/Si substrate with pre-patterned chromium-palladium electrodes. Using scanning tunnelling potentiometry, we observed that the local potential gradient (and resulting current flow) significantly changed compared with the potential gradient arising from the electrodes’ position and biasing. This could be attributed to local material ad-layers, unspecified contact or additives present between sub-layers. This finding represents an important step in understanding the effect of transfer techniques on the quality of multilayer van der Waals mono- and heterostructures.

Ultrathin All-2D Lateral Graphene/GaS/Graphene UV Photodetectors by Direct CVD Growth.

UV-sensitive lateral all-two-dimensional (2D) photodetecting devices are produced by growing the large band gap layered GaS between graphene electrode pairs directly using chemical vapor deposition methods. The use of prepatterned graphene electrode pairs on the Si wafer enables more than 200 devices to be fabricated simultaneously. We show that the surface chemistry of the substrate during GaS leads to selective growth in graphene gaps, forming the lateral heterostructures, rather than on the surface of graphene. The graphene/GaS/graphene lateral photodetecting devices are demonstrated to be sensitive to UV light only, with no measurable response to visible light. Furthermore, we demonstrate UV-band discrimination in photosensing, with measured photocurrents only produced for middle-UV and not for near-UV wavelength regions. The detection limit could reach down to 2.61 μW/cm2 with a photoresponsivity as high as 11.7 A/W and a photo gain of 53.7 under 270 nm excitation. Gate-dependent modulation of the photocurrent is also demonstrated. The photodetectors exhibit long-term stability and reproducible ON-OFF switching behavior, with a response time lower than 60 ms. These results provide insights into how ultrathin UV sensing devices can be created using only 2D materials by exploiting large band gap 2D semiconductors such as GaS.