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Dissertation Summary
Firstly, I examined the source-drain current (Id) vs. gate voltage (Vg) characteristic, i.e., the
Id-Vg, focusing on the threshold voltage of the onset of the Id-Vg curve, Vth. The Id-Vg changes
were observed in the positive Vg direction when the MoS2 channel contacts TCNQ and F4TCNQ solutions in solvents of isopropyl alcohol (IPA), acetonitrile (ACN), and dimethyl
sulfoxide (DMSO). The shift of Vth from the pure solvent condition, ΔVth, increases
monotonically with concentration in isopropanol (IPA) solvent, which can be well simulated
with Langmuir-type adsorption kinetics. I judged that the TCNQ and F4- TCNQ solutes are
partially solvated by the IPA solvent and adsorbed on the MoS2 channel. At the same time,
the saturated value of ΔVth shows a significant difference between the TCNQ and F4-TCNQ
solutes. I measured the ratio of the saturation ΔVth of F4-TCNQ to that of TCNQ, which
shows a decrease of 5.2, 4.2, 1.7, and 1.3 in the orders of the vacuum system, isopropanol
(IPA), acetonitrile (ACN), and dimethyl sulfoxide (DMSO), which coincide with the order
of the dielectric constant of these solvents of 18.0, 36.0, and 46.6, respectively. The solutes
cause the Id-Vg curve both by the charge transfer and the gating effect, the latter of which is
screened by the existence of the solvent. This study demonstrates that the solution FET can
be employed to solid-solution interface chemistry.
Secondly, I developed a biosensor using a field-effect transistor (FET) based on atomically
thin molybdenum disulfide (MoS2) for the detection of uric acid (UA) in solution
environment. I focused on demonstrating the detection of uric acid (UA), a vital marker for
diseases requiring continuous human body monitoring with in vivo conditions for medical
diagnoses. The IPA solvent was used as a solvent. When we introduce the UA solution
through the microfluidic channel, the Id-Vg plot shifts towards the left from the stabilized
pristine condition, indicating the electron donor-type behavior. I executed the VASP DFT
calculation which supports this behavior. The Id-Vg plots shift with UA concentration, whose
shift is numerically estimated by measuring the threshold voltage. The sensor behavior is
reversible, and the drain current returns to its original value when the channel is washed with
pure solvent. The results demonstrate the feasibility of applying the MoS2-FET device to the
UA detection in solution, suggesting the possible use in the solution environment.
Thirdly, I investigated the photoisomerization and thermal reset of trans- and cis-azobenzene
molecules integrated on a molybdenum disulfide (MoS2) field-effect transistor (FET) channel.
UV light was irradiated on trans-azobenzene to convert its isomer cis-azobenzene. In the
absence of light, the trans-to-cis conversion of the azobenzene molecules occurred, but it
required nearly a day to complete. However, heat conversion from the cis isomer back to the
trans-isomer was achieved within a much shorter timeframe of approximately 90 seconds.
The isomerization process was monitored through the measurement of Id-Vg of the MoS2FET device and quantitative observation was assumed from the threshold voltage
determination. By analyzing the Id-Vg shift and conducting DFT calculations, it is evident
that both trans and cis azobenzene exhibit electron-donating behavior. However, this
behavior is significantly more pronounced for cis azobenzene. The activation energy for the
conversion from cis-to-trans azobenzene molecules was estimated through the thermal
relaxation process, employing the Arrhenius equation.
Fourthly, I investigated the behavior of the MoS2-FET sensor when exposed to 2-propanol
(IPA) solvent. To achieve this, I designed a novel device featuring a film covering the MoS2
channel, and photoresist masks shielding the source and drain electrodes. Additionally, a
PDMS microfluidic channel system was fabricated to deliver the liquid to the channel. I
analyzed the impact of 2-propanol on the FET properties and observed a gradual change in
the drain current relative to the gate voltage after contact with the liquid. This change is
observed toward a positive Vg direction. DFT calculation data supports the experimental
behavior. Furthermore, I studied the effect of varying liquid flow velocities on the Id-Vg
behavior within the channel.
Finally, I investigated the sensor behavior of the MoS2-FET, focusing on the interaction with
the dopamine molecule in microfluidic environments. Dopamine detection is essential for
understanding neurological disorders, monitoring treatment effectiveness, and advancing
drug development. Both isopropyl alcohol (IPA) and water solvents were used to prepare
dopamine solutions with varying concentrations. Successful device survival was achieved
when the device was exposed to a dopamine-water solution. I measured the Id-Vg curve of
the FET with the flow of dopamine solution through the microchannel and found the
threshold voltage (Vth) shifted to the positive Vg direction. This observed behavior of Vth
indicates the electron is transferred from the MoS2 channel to the dopamine molecule, which
was further confirmed by photoluminescence spectroscopy and DFT calculation data.
In conclusion, I have effectively detected molecules in a solution environment using various
solvents. Initially, there were limitations when utilizing water as a solvent in our prepared
FET devices. However, recently, we have made a significant discovery by identifying the
optimal thickness of the TiO2 protective layer over the MoS2 channel, which has extended
the survival time even in water solvent.
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