Dynamic Observation of Retinal Response to Pressure Elevation in a Microfluidic Chamber.

Dynamic observation of cell and tissue responses to elevated pressure could help our understanding of important physiological and pathological processes related to pressure-induced injury. Here, we report on a microfluidic platform capable of maintaining a wide range of stable operating pressures (30 to 200 mmHg) while using a low flowrate (2-14 µL/h) to limit shear stress. This is achieved by forcing flow through a porous resistance matrix composed of agarose gel downstream of a microfluidic chamber. The flow characteristics were investigated and the permeabilities of the agarose with four different concentrations were extracted, agreeing well with results found in the literature. To demonstrate the capability of the device, we measured the change in intracellular Ca2+ levels of retinal ganglion cells in whole mouse retina in response to pressure. The onset of enhanced pressure results in, on average, an immediate 119.16% increase in the intracellular Ca2+ levels of retinal ganglion cells. The demonstrated microfluidic platform could be widely used to probe cell and tissue responses to elevated pressure.

Probing Light-Stimulated Activities in the Retina via Transparent Graphene Electrodes.

Graphene has triggered tremendous research due to its superior properties. In particular, the intrinsic high light transmission illustrates the unique advantage in neural biosensing. Here, we combine perforated flexible graphene electrodes with microfluidic platforms to explore real-time extracellular electrical activities of retinal ganglion cells (RGCs). Under light stimulation, the transparent graphene electrodes have demonstrated the capability of recording the electrical activities of stimulated RGCs in direct contact. Different types of RGCs have shown three distinct light induced patterns, ON, OFF, and ON-OFF, which are primarily operated by cone photoreceptors. Moreover, the observed spiking waveforms can be divided into two groups: the biphasic waveform usually occurs at contacts with soma, while the triphasic waveform is likely related to the axon. Under high K+ stimulation, the graphene electrodes exhibit higher electrical sensitivity than gold counterparts with an average 2.5-fold enhancement in spiking amplitude. Furthermore, a strong response has been observed with the firing rate first increasing and then ceasing, which could be due to the potassium-induced neural depolarization. These results show that graphene electrodes can be a promising candidate in the electrophysiology studies of retina and offer a route to engineering future two-dimensional materials-based biosensors.

Enhanced photocurrent response speed in charge-density-wave phase of TiSe2-metal junctions.

Group IVB transition metal dichalcogenides (TMDCs) have attracted significant attention due to their predicted high charge carrier mobility, large sheet current density, and enhanced thermoelectric power. Here, we investigate the electrical and optoelectronic properties of few-layer titanium diselenide (TiSe2)-metal junctions through spatial-, wavelength-, temperature-, power- and temporal-dependent scanning photocurrent measurements. Strong photocurrent responses have been detected at TiSe2-metal junctions, which is likely attributed to both photovoltaic and photothermoelectric effects. A fast response time of 31 µs has been achieved, which is two orders of magnitude better than HfSe2 based devices. More importantly, our experimental results reveal a significant enhancement in the response speed upon cooling to the charge-density-wave (CDW) phase transition temperature (TCDW = 206 K), which may result from dramatic reduction in carrier scattering that occurs as a result of the switching between the normal and CDW phases of TiSe2. Additionally, the photoresponsivity at 145 K is up to an order of magnitude higher than that obtained at room temperature. These fundamental studies not only offer insight for the photocurrent generation mechanisms of group IVB TMDC materials, but also provide a route to engineering future temperature-dependent, two-dimensional, fast electronic and optoelectronic devices.

Ultrafast Photocurrent Response and High Detectivity in Two-Dimensional MoSe2-based Heterojunctions.

Two-dimensional (2D) transition metal dichalcogenide (TMDC) materials have garnered great attention on account of their novel properties and potential to advance modern technology. Recent studies have demonstrated that TMDCs can be utilized to create high-performing heterostructures with combined functionality of the individual layers and new phenomena at these interfaces. Here, we report an ultrafast photoresponse within MoSe2-based heterostructures in which heavily p-doped WSe2 and MoS2 flakes share an undoped MoSe2 channel, allowing us to directly compare the optoelectronic properties of MoSe2-based heterojunctions with different 2D materials. Strong photocurrent signals have been observed in both MoSe2-WSe2 and MoSe2-MoS2 heterojunctions with a photoresponse time constant of ∼16 µs, surmounting previous MoSe2-based devices by three orders of magnitude. Further studies have shown that the fast response is independent of the integrated 2D materials (WSe2 or MoS2) but is likely attributed to the high carrier mobility of 260 cm2 V-1 s-1 in the undoped MoSe2 channel as well as the greatly reduced Schottky barriers and near absence of interface states at MoSe2-WSe2/MoS2 heterojunctions, which lead to reduced carrier transit time and thus short photocurrent response time. Lastly, a high detectivity on the order of ∼1014 Jones has been achieved in MoSe2-based heterojunctions, which supersedes current industry standards. These fundamental studies not only shed light on photocurrent generation mechanisms in MoSe2-based heterojunctions but also open up new avenues for engineering future high-performance 2D optoelectronic devices.

In situ monitoring of electrical and optoelectronic properties of suspended graphene ribbons during laser-induced morphological changes.

Exploring ways to tune and improve the performance of graphene is of paramount importance in creating functional graphene-based electronic and optoelectronic devices. Recent advancements have shown that altering the morphology of graphene can have a pronounced effect on its properties. Here, we present a practical and facile method to manipulate the morphology of a suspended graphene ribbon using a laser to locally induce heating while monitoring its electrical and optoelectronic properties in situ. Electrical measurements reveal that the conductance of suspended graphene transistors can be tuned by modifying its morphology. Additionally, scanning photocurrent measurements show that laser-induced folded graphene ribbons display significantly enhanced localized photocurrent responses in comparison with their flat counterparts. Moreover, the localization of the laser-induced heating allows for a series of folds to be induced along the entire graphene ribbon, creating targeted photocurrent enhancement. Through further investigations, it is revealed that the photo-thermoelectric effect is the primary mechanism for the increased photocurrent response of the device. Our experimental results explore the mechanisms and consequences of the folding process as well as provide a strategy to manipulate morphology and physical properties of graphene for future engineering of electronics and optoelectronics.

Near-infrared optical transitions in PdSe2 phototransistors.

We investigate electronic and optoelectronic properties of few-layer palladium diselenide (PdSe2) phototransistors through spatially-resolved photocurrent measurements. A strong photocurrent resonance peak is observed at 1060 nm (1.17 eV), likely attributed to indirect optical transitions in few-layer PdSe2. More interestingly, when the thickness of PdSe2 flakes increases, more and more photocurrent resonance peaks appear in the near-infrared region, suggesting strong interlayer interactions in few-layer PdSe2 help open up more optical transitions between the conduction and valence bands of PdSe2. Moreover, gate-dependent measurements indicate that remarkable photocurrent responses at the junctions between PdSe2 and metal electrodes primarily result from the photovoltaic effect when a PdSe2 phototransistor is in the off-state and are partially attributed to the photothermoelectric effect when the device turns on. We also demonstrate PdSe2 devices with a Seebeck coefficient as high as 74 µV K-1 at room temperature, which is comparable with recent theoretical predications. Additionally, we find that the rise and decay time constants of PdSe2 phototransistors are ∼156 µs and ∼163 µs, respectively, which are more than three orders of magnitude faster than previous PdSe2 work and two orders of magnitude over other noble metal dichalcogenide phototransistors, offering new avenues for engineering future optoelectronics.