5 1.4 (a) The distribution of the bandgap of 2D material and the corresponding. b) The ON/OFF ratio and mobility of 2D material-based photodetector [3]. 27 2.7 (a) Optical and (b) photocurrent images of a folded graphene ribbon with. c) The gate-dependent scanning photocurrent image whenVGvaries from 1.5V to 2.2V.
Two-Dimensional Materials
Photodetector
In the case of PVE (Figure 1.3a), when the photon energy of the incident light is larger than the bandgap of the semiconductor, electrons in the valence band can be excited to the conduction band, producing photo-excited electron-hole pairs (EHPs). One of the key characteristics of TMDCs is that the bandgap can be tuned by changing the number of layers and the chemical composi- tion of the material [37] [38].
Electrical and Scanning Photocurrent Measurements
To achieve high electrical resolution, a 1000:1 voltage divider is applied to the output channel of the DAQ card. A third electrode connected to the DAQ board can be used as a gate to introduce modulation into the device's photocurrent response.
Graphene Based Field Effect Transistor
Gate-dependent measurements can be used to obtain the appropriate transconductance and calculate the mobility of the photodetector. In addition, polarization-dependent photocurrent measurement can be used to study the network structure and anisotropic behavior of the channel material.
Two-dimensional Materials-based Biosensing
In addition, temperature-dependent measurements can be used to determine the effect of temperature on photocurrent. Gate-dependent photocurrent measurements can be performed to study photocurrent generation mechanisms.
Organization of the Dissertation
Za′(f) +Ze′(f) (1.2) where Za′ represents the impedance of the recording system and Ze′ illustrates the impedance of the microelectrode. A signal such as a gate voltage can modulate the conductance of a graphene channel through the gate capacitance, in which the output current is related to the transconductance for a given gate voltage and gate input signal (Figure 1.7c).
Introduction
In this work, we report a practical and versatile method to manipulate the electrical and optoelectronic properties of graphene by locally modifying the morphology of a suspended graphene ribbon via a laser beam. These experimental results provide a facile method to alter the morphology of graphene as an effective approach to manipulate its electrical and optoelectronic properties.
Graphene Transistor Fabrication and Characterization
After the wet etching process, the symmetric 2D band and small D band (~1350 cm−1) suggest that the pristine quality of the graphene ribbon is retained. Proper control of the flow rate between CH4 and H2 is crucial in regulating the number of graphene layers produced.
In situ Monitoring of Electrical and Optoelectronic Properties of Folded
Electrical and optoelectronic properties were investigated before changing the morphology of the hanging graphene ribbon. The optical and photocurrent images of the flat graphene ribbon are shown in Figures 2.4a and 2.4b, respectively.
Folded Graphene Structure
To further understand these folding mechanisms, we estimate the local temperature distribution along the suspended graphene ribbon. The temperature distribution along the graphene ribbon can be obtained from the following heat diffusion equation in the cylindrical coordinate.
Photocurrent Generation Mechanism
The photovoltage established across the device can be determined from Vpc=IpcR, where Ipc is the line intersection across the green dashed line in Figure 2.7c and R is the gate-dependent resistance obtained from the electrical transport measurements (Figure 2.7f). . VG=VG−VDirac, ℏ is the plank constant, the vF is the Fermi velocity and CG is the.
Conclusion
It is clear that Vpc has an almost similar gate-dependent pattern to the calculated Seebeck coefficient, suggesting that photocurrent signals in folded structures are mainly attributed to the PTE [34] [92]. Recently, PTE-induced photocurrent responses have also been observed at single-bilayer graphene junctions and TMDC-metal junctions due to the Seebeck coefficient difference across these junctions [90] [93].
Introduction
This allows more K+ ions to flow out of the cell, leading to a repolarization of the membrane potential. It is the result of a rapid depolarization and repolarization of the cell membrane and is characterized by a rapid and large change in the membrane potential.
Fabrication and Characterization of Transparent Graphene Electrodes
The copper foil was annealed at 1000°C with 100 sccm of hydrogen for 1 h after the system was pumped to 10 mTorr. 7] (b) Raman spectrum of graphene grown under 532 nm illumination. c) Fabrication procedure of flexible graphene probes.
In vitro Retina Recordings with Transparent Graphene Electrodes
The zoomed-in view of a single peak indicates that it can be attributed to the extracellular action potential of an RGC body (Figure 3.5d).
Light Induced Stimulation to Retina
In addition, different electrical spike waveforms were recorded by transparent graphene electrodes (Figure 3.8), likely related to various cell morphologies, synaptic connectivity, and relative position to the electrode. After the action potential travels away from the soma to the axon, the cell body acts as the source, resulting in a positive voltage (Figure 3.8c).
Extracellular High K + Concentration Stimulation
Both the light and dark gray boxes represent RGCs with the charge distributed inside and outside the cell membrane, while the dark box highlights the position that the action potential is located. The orange indicator illustrates an external recording electrode while the black arrow shows the direction of propagation of the action potential on the soma and axon.
Conclusion
The middle line and the small square in the box represent the median and mean values, respectively. 1 This chapter is adapted from Probing Light-Stimulated Activities in the Retina via Transparent Graphene Electrodes.
Introduction
In addition to the negative pressure fluidic channels, the sensor design includes a 200µm wide local delivery channel, which allows for local chemical stimulations and the study of the corresponding response in the retina. The thickness of the microfluidic channel is approximately 100µm, with the top and bottom channels representing the negative pressure channels and the middle chamber serving as a negative pressure and local delivery channel.
Ex vivo Retina Recordings with Single Channel µ pMEA Device
This indicates the importance of the negative pressure applied by the vacuum channel to improve the contact between the retinal tissue and electrodes. Simulation results indicate that reducing the thickness of the vacuum channel from 2mm to 100µm can increase the pressure difference between the holes closest to the vacuum channel and the farthest away from the vacuum channel from about 3% to ~122%.
Ex vivo Retina Recordings with Two-suction Channel µ pMEA Devices
The position of the retina relative to the local delivery channel is another key factor in this experiment. Any bubble that appears in the local delivery path can block the local delivery channel and stop the continuous flow of media.
Gap Junction Contribution to Intercellular RGC Communications
This provides more precise control over the flow of the medium and helps prevent air bubbles from entering the local delivery channel. It was observed that the electrodes located outside the local delivery channel exhibited a shorter refractory period, on average 25.2 seconds, compared to the electrode located within the delivery channel.
Graphene-based High-resolution Imaging
For global chemical stimulation, in which stimulating reagents are applied to the retina from the superior well, Figures 4.8a and 4.8b show the mean action potential waveform. The fluorescence image clearly shows a single RGC and a bundle of axons on top of the graphene electrode, which is likely related to the recorded waveform.
Conclusions
Flexible G-FETs for Electrophysiological Studies of Retina
As a result, the graphene is always modulated by the previous state of the gate voltage. Third, more pronounced photocurrent patterns were observed in the middle region of the graphene ribbon.
Interactions between Graphene and Cell membrane
Introduction
The first is the scattering force, which is proportional to the intensity of the light and acts in the direction of the incident light. The other is the gradient force, which is induced by the uneven light field and has a direction along the gradient of the light intensity.
Graphene Multi-ribbon Transistors for Studying Cells
In particular, the corresponding photocurrent image can help to locate the position of the multiple graphene ribbon. The conductivity of the multi-graphene tape recovered and returned to a value similar to the initial state (green line in Figure 5.7g).
Conclusions
Introduction
167], while 2D 1T-TaS2 has also shown electric field-sensitive characteristics and CDW phase transition upon application of a large bias [168]. Most importantly, temperature-, power-, and wavelength-dependent photocurrent measurements reveal a step change at the phase transition temperature (170 K).
Device Fabrication
We note that although TaTe2 has been known to experience a CDW phase transition at 170 K (TCDW) with lattice distortion associated with the CDW phase (as shown in the inset of Figure 6.1c, no significant electrical resistance change can be observed at this temperature, which is consistent with previous results for TaTe2[176].The inset diagram illustrates the prismatic metallic structure and the distortion lattice in the CDW phase.
Result and Discussion
The pronounced photocurrent change at the TaTe2 junctions is due to both PTE and the photovoltaic effect (PVE), which are related to the dislocation of the atomic arrangement at the TaTe2-metal interface [174]. Indeed, both factors depend on the reconstruction of the local electronic structure and lattice distortion, leading to a more obvious change in the photocurrent than the electrical resistance. The laser power is 1.76 mW for all photocurrent images. b) Spatial distribution of photocurrent generated as a function of laser position of TaTe2 metal compounds at 130 K. The black dot profile and solid red curve represent recorded photocurrent data and corresponding Gaussian fit. The blue arrow marks the "tail" of the photocurrent on the electrode area. The yellow backgrounds represent the electrode area. c) Temperature-dependent photocurrent measurement indicates a step-like transition from the normal metal phase to the CDW phase, illustrating an increasing photocurrent with decreasing temperature. To further investigate the optoelectronic characteristics of the CDW phase transition of TaTe2, we measure the photocurrent response under a 1064 nm laser versus the light intensities as a function of temperature (Figure 6.3a).
Conclusions
Summary
In Chapter 6, we explore the optoelectronic response of TaTe2 as a metal-transition metal dichalcogenide material when it undergoes charge density wave (CDW) phase transition. Compared to the temperature-dependent electrical resistance measurement commonly used in the literature, the optoelectronic response during CDW phase transition exhibits more significant features of CDW phase transition.
Future Outlook
The high electrical conductivity and small size of graphene probes enable them to record neural activity with high spatial and temporal resolution. Overall, the potential applications of flexible graphene probes extend beyond neuroscience research, including biosensing, wearable devices, energy storage, and flexible electronics.
Recipe of Chemical Vapor Deposition Growth Graphene Growth
After the growth process is completed, the boat is carefully removed from the hot zone and the furnace is turned off. When the system temperature drops to 200 °C, the valve slowly closes and the vacuum pump is switched off.
Bubbling-assisted Graphene transfer
The end of one of the pipe connections is loosened, allowing the Ar gas to purge the entire system. Transfer graphene to the target substrate and dry the device naturally overnight to remove any water residue and improve the graphene's adhesion to the substrate.
Suspended Graphene Ribbon Device Fabrication
Flexible Graphene Probes Fabrication
Retina Recipe
Recent advances in electronic and optoelectronic devices based on two-dimensional transition metal dichalcogenides. Microscopic evidence for strong periodic lattice distortion in two-dimensional charge-density wave systems.
