12 Figure 1.11 (a) The arrangement of carbon atoms in graphene and (b) the band structure of graphene in k-space. 73 Figure 4.4 Schematic representation of a cell coupled to a BIT-FET (left) and the variation in device conductance G (right).
Carbon Nanotube
Growth Mechanism
As shown in Figure 1.2a, decomposed carbon diffuses down through the metal catalyst until there is excess carbon for growth. In Figure 1.3, with the increasing ratio of methanol/ethanol, a higher percentage of semiconducting SWNTs is obtained in the growth result [12].
![Figure 1.2 Growth mechanism of CNTs: (a) tip-growth model and (b) base-growth model. Image (a) and (b) reprinted with permission from Ref [8]](https://thumb-ap.123doks.com/thumbv2/123dok/10731506.0/17.918.263.651.255.527/figure-growth-mechanism-growth-growth-image-reprinted-permission.webp)
Large-scale Circuits
Graphene
Monolayer Films in Centimeters
Second, many metal platforms such as platinum (Pt), cobalt (Co), nickel (Ni), and copper (Cu) have been used to synthesize high-quality graphene. As shown in Figure 1.13, the domain size of monolayer graphene can be as large as 1.5 inches under an optimized growth condition.
Switching Dirac Point
Most studies have found that the on-off current ratio of a graphene transistor is less than 10, a few orders of magnitude lower than that of a semiconductor nanotube transistor [78]. As discussed earlier, photocurrent can be generated due to a local in-plane electric field in the graphene transistor.
Toward Touch Screens
Scanning photocurrent microscopy can also be used to study the unique optical properties and possible applications of graphene. In Figure 1.16, the largest photocurrent of about ± 15 nA occurs near the metal contacts, indicating a strong local electric field near the contacts [60].
Molybdenum disulfide
Progress in Preparation
Typically, several main types of Mo-based precursors are involved in the preparation to grow monolayer or single-layer MoS2 films. Moreover, a recent approach based on metal-organic chemical vapor deposition (MOCVD) has been used to grow TMDC layers [102].
Tuning On-off Current Ratio and Photocurrent
When there is a positive VDS, the band bends at the source contact, leading to an increase in photocurrent.
Flexible and Ultrasensitive Devices
CONTROLLING GROWTH OF CARBON-BASED MATERIALS
CVD Synthesis of Graphene
- Monolayer and Few-layer Graphene
- Graphene Transistors
When the gate voltage was swept from 0 V to 1.6 V (black curve) or in the opposite direction (red curve), the conductivity of a graphene transistor was recorded at a source-drain voltage of 50 mV (Figure 2.4b). At the same time, the light reflected from the device was recorded by a Si photodetector, which can be superimposed with the photocurrent image to find the position of the photocurrent response (Figure 2.5d).
Ultrathin SWNT Network Framed Graphene Hybrids
- Motivation
- Experimental Methods
- Results and Discussion
INTEGRATION OF ALIGNED SWNTs AND MoS 2
CVD Synthesis of Aligned SWNTs
- SEM, Raman and AFM Characterizations
- Electronic and Optoelectronic Properties
Finally, the catalyst patterns in the form of lines remained on the surface after the Scotch tape was removed (Figure 3.1a). Atmospheric growth was first followed by a flow of 200 sccm H2 at 685 ºC for 30 minutes and later with an additional flow of 50 sccm Ar through one alcohol bubbler at a higher temperature of 815 ºC for a further 30 minutes (Figure 3.1b). On a small scale, we can estimate that the density of SWNT films is about 1~5 nanotubes/µm (Figure 3.3b).
From the profile in Figure 3.5b, we can estimate that the diameter of SWNTs varies from 0.7 nm to 2.2 nm. After we obtain the aligned SWNT film, we transfer it to a transistor where the distance between two prepatterned electrodes is 5 µm (Figure 3.6a). As shown in Figure 3.6b, when the gate voltage swept from -3 V to 4 V, the conductance of horizontally aligned SWNTs was collected at a source-drain bias of 50 mV.
As shown in Figure 3.7a, when the gate voltage went from -3 V to 3 V, the conduction curve still shows a p-type characteristic at zero gate bias. As shown in Figure 3.10a, at a gate voltage between -1 V and 1 V, the conduction curve shows a consistent p-type characteristic of SWNTs. Moreover, we can observe the bundled SWNTs from the reflection signals (Figure 3.10c), which later helps us in the investigation of SWNT-MoS2 hybrids.
CVD Synthesis of Monolayer MoS 2
- Raman and Photoluminescence Spectra
- CVD-grown Single-layer MoS 2 Transistor
Raman microscopy can be used to investigate many properties of MoS2, such as thickness and photoluminescence. To begin with, we performed Raman spectrum mapping of triangular MoS2 films with a side length of ~ 8 µm (Figure 3.13a). In the optical image, we can see some dots in the center of these triangular films in Figure 3.12c and Figure 3.13a, which can be seen as seeds that help the growth of uniform MoS2 films with a large surface area [98].
In addition, we plot the spectra in terms of the E12g mode, the A1g mode, and the distance between two modes, as shown in Figure 3.13c and Figure 3.13d. Not only can we see Raman scattering to examine the thickness and quality of MoS2 film, we can also perform photoluminescence spectroscopy at the four locations shown in Figure 3.13a. As shown in Figure 3.14a, the spectrum peak is observed at ~673 nm, corresponding to 1.84 eV photon energy, which is consistent with reported results [150].
Similarly, a photoluminescence mapping has been performed for this film in Figure 3.14d, indicating a uniform monolayer MoS2 film except for the multilayer in the middle with a relatively low photoluminescence intensity. These results are consistent with our previous Raman spectra, which imply the uniform distribution of CVD-grown MoS2 films. As shown in Figure 3.15a and Figure 3.15b, we can observe strong photoresponses at the junctions between metal and MoS2 films [151].
Probing Aligned SWNTs Doped Ultrathin MoS 2
- Motivation
- Experimental Methods
- Results and Discussion
GRAPHENE OPTOELECTRONIC PROBES
Existing Approaches
Several variations of this technique can be used to study the behavior of individual ion channels, depending on the suction performed through a microelectrode to attach a piece of plasma membrane. If more suction is applied, a small portion of the membrane at the tip of the electrode may move, leaving the electrode sealed with the rest of the cell. This "whole-cell" mode allows very stable intracellular recording, as shown in Figure 4.1a and Figure 4.1b.
From the patch-clamp recording, the membrane potential is about −70 mV at rest and about 20 mV when depolarized. Optical imaging, such as voltage-sensitive dyes (VSDs) and calcium sensors, allows neuroscientists to simultaneously obtain information about the source of neural activity as well as its time course, making it possible to study neural activity and related functions in the brain. As illustrated in Figure 4.2, for different dyes such as GCaMP6s and GCaMP6f, simultaneous fluorescence dynamics were recorded by an EMCCD camera while electrical spikes were stimulated through patch-clamp microelectrodes.
However, the main disadvantage of calcium sensors or VSDs, in the common techniques of optical imaging, is their electrical sensitivity, which can be several orders of magnitude lower than patch-clamp techniques in electrophysiology. Furthermore, microelectrode arrays (MEAs) that serve as neural interfaces stimulate or connect neurons in the electronic circuit. For example, as shown in Figure 4.3a, MEAs have been used as an effective tool to detect acute neurotoxicity [182].
Graphene Biosensors
Researchers have applied a novel approach with a SiO2 nanotube as a FET to bring contact with cells, which allows the recording of the intracellular transmembrane potential [184]. The SiO2 nanotube can stably record the full-amplitude intracellular action potential when it penetrates the cell membrane to form a tight seal (Figure 4.4). Their studies on cardiomyocyte cells show that FETs can be applied as a new measurement tool to investigate cell membrane potentials.
One unique advantage of graphene is its exposed volume to the environment, which maximizes sensitivity to local change. Graphene ribbons with pores in nanometer size were fabricated to record the ionic activities and detect the DNA translocations simultaneously [185] (Figure 4.5a). Not only is graphene well compatible for nanoscale biomolecules, but it can also be adaptable to large-scale detection of target analytes.
Graphene-based microelectrode arrays have been developed for application to hippocampal slices, which enables the recording of high-frequency burst activity and slow synaptic potentials [188] (Figure 4.5c). Furthermore, single-layer graphene can transmit more than 97% of incident light [189], making it compatible with optical imaging. To summarize, all of graphene's unique properties make it an ideal candidate for detecting electrical and electrochemical signals in neuronal research.
Experimental Methods
- Device Fabrication and Characterization
- Cell Loading and Imaging
The ultrafast nature of the graphene photocurrent response enables the decoding of a single waveform that can coincide with action potentials from spontaneous bursts of individual synapses and spines with a time resolution of tens of microseconds. Importantly, we show that the 2D nature of graphene enables seamless recording of millivolt extracellular potential changes of an entire neural network composed of randomly distributed synapses/spines. After annealing in the presence of 200 sccm of hydrogen at 1000 °C for 1 h, the hydrogen flow was reduced to 100 sccm and 20 sccm of methane was introduced into the system to complete the graphene growth within 30 min.
Then, the PMMA layer was spun on top of the copper foil to hold the graphene film. The PMMA-graphene film was separated from the copper foil by a wet etching process in ferric chloride solution, and then transferred onto a pre-patterned cover slip. We then performed Raman spectroscopy measurements to inspect the quality and thickness of graphene on the coverslip with a 532 nm laser.
The devices were then incubated at 37°C to allow neurons to adhere to the PLL-coated glass surfaces. The uneven volumes in the reservoirs created a passive pressure difference, which generated a steady flow of media from the glial chambers to the adjacent neuronal chambers, thus supplying the neurons with fresh glia-conditioned medium. Finally, fresh neuron media was added to the devices every 24 hours and the collected media was simultaneously removed from the waste reservoirs.
Results and Discussion
SUMMARY AND FUTURE WORK
Summary
In this thesis, three attractive nanomaterials, CNT, graphene and MoS2, have been thoroughly summarized in the aspects of structures, growth mechanisms, electronic/optoelectronic properties and extensive applications. In CHAPTER 2 we describe the synthesis of high-quality and uniform nanomaterials using advanced transfer techniques. These ultrathin CNT graphene films not only have improved mechanical strength (Figure 2.6c), but also high optical transmittance and good electrical conductivity.
In CHAPTER 3, we studied the synthesis of SWNTs and the useful potential of aligned SWNT transistors. This new hybrid can be easily visualized under a microscope (Figure 3.16a) and shows increased Raman intensity at junctions (Figure 3.16c-e). The work presented in CHAPTER 2 and CHAPTER 3 provides new avenues for the development of hybridized structures towards future optoelectronic applications.
We have introduced many exciting breakthroughs in the applications of CNT, graphene and MoS2 due to their excellent properties. In CHAPTER 4, the flexibility and compatibility of graphene, integrated with scanning photocurrent microscopy, allows us to study the electrical and electrochemical signals of neuron synapses. The detailed waveform of signals for neurons at rest and during depolarization can be decoded in real time and consistent with image microscopy by flowing different K+ concentrations in our microfluidic systems (Figure 4.10 and Figure 4.11).
Future Work
In conclusion, the current studies of nanomaterials have created many new properties and practical applications, even though many challenges still exist. Hopefully, the work presented in this thesis will draw attention to future development of nanomaterials and nanotechnology.
Recipe of 2D Graphene Growth
Recipe of 3D Graphene Growth
Recipe of Aligned SWNT Growth
Recipe of Monolayer MoS 2 Growth
Transistor Fabrication
Nanotube Imaging with Sulfur Molecules
Calculation of Local Potential Changes via Scanning Photocurrent Measurements
