Progress in bionic sensing and bio-electronic interfacing requires a thorough understanding of electronic and ionic transport in functional bio-compatible materials. The well-known polymer mixture poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) is central to many optoelectronic applications which necessitate flexible, transparent biocompatible materials. While PEDOT:PSS is traditionally studied for its hole transport properties and resistive H2O response, research interest has recently turned to ionic transport in the context of bio-electronic interfacing and sensing. Cations Na+ and K+ are often present in PEDOT:PSS as byproducts from industrial production. Uptake of water into PEDOT:PSS disrupts hydrogen bonding that maintains rigidity of the PSS matrix, causing film swelling and hydronium production due to water interaction at the hydrophilic SO3- moiety. As water uptake increases, mobility of Na+, K+, H3O+ and other ions may become hindered due to the formation of electrical double layers and hydration shells around ions. At the same time, water-induced swelling of the PSS matrix increases the distance between adjacent conductive PEDOT domains, which reduces electronic mobility. Despite its widespread use as a hole transport material in a variety of organic optoelectronic devices, the effect of water on electronic and ionic transport in PEDOT:PSS remains unclear. To probe both electronic and ionic response during water uptake, we perform ultra-wide range dielectric spectroscopy from sub-Hz to optical frequencies while changing humidity conditions in a controlled environment. We correlate the frequency-dependent measurements with those of a PEDOT:PSS-coated quartz crystal microbalance (QCM) to estimate the mass of adsorbed H2O in the film. We show that the presence of water has an effect on electronic and ionic mobility in the film and both electronic and ionic transport play a role in defining the optoelectornic properties of PEDOT:PSS under a wide range of humidities.
Functional assemblies of materials can be realized by tuning the work function and band gap of existing materials. Here we demonstrate the structural assembly of two- and three-dimensional (2-D) and (3-D) nanomaterials and investigate the optical and electronic properties of an assembly of monolayer WS2 on a rough polycrystalline NiO surface. Monolayer WS2 (2-D material) was transferred onto the NiO surface using a polymer-assisted transfer technique and resulted in a surface roughness about 30× greater than that of WS2 on SiO2. Raman maps of WS2 transferred onto NiO display a spatial nonuniformity of the E2g1 (∼352 cm−1) and A1g (∼418 cm−1) peak intensities, indicating that regions of the WS2 exist in a strained condition on the 3-D NiO surface. Kelvin probe force microscopy measurements show that the WS2-SiO2 assembly has a surface potential 62±5 mV lower than that of SiO2, whereas that of WS2-NiO is 11±5 mV higher than NiO, indicating that a monolayer of WS2 is sufficient to modify the surface potential by acting as either an electron donor or acceptor with the underlying surface. Thus, 2-D and 3-D materials can be organized into functional assemblies with electron flow controlled by the WS2 either as the electron donor or acceptor.
Copper phthalocyanine (CuPc) is an important hole transport layer for organic photovoltaics (OPVs), but interaction with ambient gas/vapor may lead to changes in its electronic properties and limit OPV device lifetimes. CuPc films of thickness 25 and 100 nm were grown by thermal sublimation at 25°C, 150°C, and 250°C in order to vary morphology. We measured electrical resistance and film mass in situ during exposure to controlled pulses of O2 and H2O vapor. CuPc films deposited at 250°C showed a factor of 5 higher uptake of O2 as detected by a quartz crystal microbalance (QCM), possibly due to the formation of β-CuPc at T>200°C which allows higher O2 mobility between stacked molecules. While weight-based measurements stabilize after ∼10 min of gas exposure, resistance response stabilizes over times >1 h, suggesting that mass change occurs by rapid adsorption at active surface sites whereas resistive response is dominated by slow diffusion of adsorbates into the bulk film. The 25 nm films exhibit higher resistive response than 100 nm films after an hour of O2/H2O exposure due to fast analyte diffusion down to the film/electrode interface. We found evidence of decoupling of CuPc from the gold-coated QCM crystal due to preferential adsorption of O2/H2O molecules on gold.
Proliferation of environmental sensors for internet of things (IoT) applications has increased the need for low-cost platforms capable of accommodating multiple sensors. Quartz crystal microbalance (QCM) crystals coated with nanometer-thin sensor films are suitable for use in high-resolution (~1 ng) selective gas sensor applications. We demonstrate a scalable array for measuring frequency response of six QCM sensors controlled by low-cost Arduino microcontrollers and a USB multiplexer. Gas pulses and data acquisition were controlled by a LabVIEW user interface. We test the sensor array by measuring the frequency shift of crystals coated with different compositions of polymer composites based on poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) while films are exposed to water vapor and oxygen inside a controlled environmental chamber. Our sensor array exhibits comparable performance to that of a commercial QCM system, while enabling high-throughput 6 QCM testing for under $1,000. We use deep neural network structures to process sensor response and demonstrate that the QCM array is suitable for gas sensing, environmental monitoring, and electronic-nose applications.
Copper phthalocyanine (CuPc) films of thickness 25 nm and 100 nm were grown by thermal sublimation at 25°C, 150°C, and 250°C in order to vary morphology. Using a source-measure unit and a quartz crystal microbalance (QCM), we measured changes in electrical resistance and film mass in situ during exposure to controlled pulses of O2 and H2O vapor. Mass loading by O2 was enhanced by a factor of 5 in films deposited at 250°C, possibly due to the ~200°C CuPc α→β transition which allows higher O2 mobility between stacked molecules. While gas/vapor sorption occurred over timescales of < 10 minutes, resistance change occurred over timescales < 1 hour, suggesting that mass change occurs by rapid adsorption at active surface sites, whereas resistive response is dominated by slow diffusion of adsorbates into the film bulk. Resistive response generally increases with film deposition temperature due to increased porosity associated with larger crystalline domains. The 25 nm thick films exhibit higher resistive response than 100 nm thick films after an hour of O2/H2O exposure due to the smaller analyte diffusion length required for reaching the film/electrode interface. We found evidence of decoupling of CuPc from the gold-coated QCM crystal due to preferential adsorption of O2/H2O molecules on gold, which is consistent with findings of other studies.
Functional assemblies of materials can be realized by tuning the work function and band gap of nanomaterials by rational material selection and design. Here we demonstrate the structural assembly of 2D and 3D nanomaterials and show that layering a 2D material monolayer on a 3D metal oxide leads to substantial alteration of both the surface potential and optical properties of the 3D material. A 40 nm thick film of polycrystalline NiO was produced by room temperature rf-sputtering, resulting in a 3D nanoparticle assembly. Chemical vapor deposition (CVD) grown 10-30 μm WS2 flakes (2D material) were placed on the NiO surface using a PDMS stamp transfer technique. The 2D/3D WS2/NiO assembly was characterized using confocal micro Raman spectroscopy to evaluate the vibrational properties and using Kelvin probe force microscopy (KPFM) to evaluate the surface potential. Raman maps of the 2D/3D assembly show spatial non-uniformity of the A1g mode (~418 cm-1) and the disorder-enhanced longitudinal acoustic mode, 2LA(M) (~350 cm-1), suggesting that the WS2 exists in a strained condition on when transferred onto 3D polycrystalline NiO. KPFM measurements show that single layer WS2 on SiO2 has a surface potential 75 mV lower than that of SiO2, whereas the surface potential of WS2 on NiO is 15 mV higher than NiO, indicating that WS2 could act as electron donor or acceptor depending on the 3D material it is interfaced with. Thus 2D and 3D materials can be organized into functional assemblies with electron flow controlled by the WS2 either as the electron donor or acceptor.
The spatially resolved electrical response of polycrystalline NiOx films, composed of 40 nm crystallites, was investigated under different relative humidity (RH) levels. The topological and electrical properties (surface potential and resistance) were characterized with sub-25-nm resolution using Kelvin probe force microscopy and conductive scanning probe microscopy under argon atmosphere with 0%, 50%, and 80% RH. The dimensionality of surface features obtained through autocorrelation analysis of topological maps increased linearly with increased RH, as water was adsorbed onto the film surface. Surface potential decreased from 280 to 100 mV and resistance decreased from 5 GΩ to 3 GΩ, in a nonlinear fashion when RH was increased from 0% to 80%. Spatially resolved surface potential and resistance of the NiOx films was found to be heterogeneous throughout the film, with distinct surface features that grew in size from 60 to 175 nm at 0% and 80% RH levels, respectively. The heterogeneous character of the topological, surface potential, and resistance properties of the polycrystalline NiOx film observed under dry conditions decreased with increased RH, yielding nearly homogeneous surface properties at 80% RH, suggesting that the nanoscale potential and resistance properties converge with the mesoscale properties as water is adsorbed onto the NiOx film.
Interaction between ultraviolet (UV) light and carbon nanotube (CNT) networks plays a central role in gas adsorption, sensor sensitivity, and stability of CNT-based electronic devices. To determine the effect of UV light on sorption kinetics and resistive gas/vapor response of different CNT networks, films of semiconducting single-wall nanotubes (s-SWNTs), metallic single-wall nanotubes, and multiwall nanotubes were exposed to O2 and H2O vapor in the dark and under UV irradiation. Changes in film resistance and mass were measured in situ. In the dark, resistance of metallic nanotube networks increases in the presence of O2 and H2O, whereas resistance of s-SWNT networks decreases. UV irradiation decreases the resistance of metallic nanotube networks in the presence of O2 and H2O and increases the gas/vapor sensitivity of s-SWNT networks by nearly a factor of 2 compared to metallic nanotube networks. s-SWNT networks show evidence of delamination from the gold-plated quartz crystal microbalance crystal, possibly due to preferential adsorption of O2 and H2O on gold. UV irradiation increases the sensitivity of all CNT networks to O2 and H2O by an order of magnitude, which demonstrates the importance of UV light for enhancing response and lowering detection limits in CNT-based gas/vapor sensors.
The spatially resolved electrical response of polycrystalline NiO films composed of 40 nm crystallites was investigated under different relative humidity levels (RH). The topological and electrical properties (surface potential and resistance) were characterized with sub 25nm resolution using Kelvin probe force microscopy (KPFM) and conductive scanning probe microscopy under argon atmosphere at 0%, 50%, and 80% relative humidity. The dimensionality of surface features obtained through autocorrelation analysis of topological maps increased linearly with increased relative humidity, as water was adsorbed onto the film surface. Surface potential decreased from about 280mV to about 100 mV and resistance decreased from about 5 GΩ to about 3 GΩ, in a nonlinear fashion when relative humidity was increased from 0% to 80%. Spatially resolved surface potential and resistance of the NiO films was found to be heterogeneous throughout the film, with distinct domains that grew in size from about 60 nm to 175 nm at 0% and 80% RH levels, respectively. The heterogeneous character of the topological, surface potential, and resistance properties of the polycrystalline NiO film observed under dry conditions decreased with increased relative humidity, yielding nearly homogeneous surface properties at 80% RH, suggesting that the nanoscale potential and resistance properties converge with the mesoscale properties as water is adsorbed onto the NiO film.
Carbon nanotube (CNT) films composed of semiconducting single wall nanotubes (s-SWNTs), metallic single wall nanotubes (m-SWNTs), and multiwall nanotubes (MWNTs) were exposed to O2 and H2O vapor in the dark and under UV irradiation. Changes in the film conductivity and mass were measured in situ. We find that UV irradiation increases the resistive response of CNT films to O2 and H2O by more than an order of magnitude. In m-SWNT and MWNT films, UV irradiation changes the sign of the resistive response to O2 and H2O by generating free charge carriers. S-SWNTs show the largest UV-induced resistive response and exhibit weakening of van der Waals interactions with the QCM crystal when exposed to gas/vapor.
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