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Open Access Publications from the University of California

Scholarly Works (4 results)

  • Thesis
  • Peer Reviewed
UC Riverside Electronic Theses and Dissertations (2019)

Two-dimensional (2D) nanomaterials, such as graphene, hold great potential for next-generation electronics owing to their unique properties such as high carrier mobility, fast response time and high transparency. However, the absence of an electrical band gap in graphene limits its applications as a field-effect transistor (FET). To overcome this limitation, other 2D nanomaterials such as transitional metal dichalcogenides (TMDs) have been studied and attracted significant research interests. TMDs consist of 2D stacked layers of covalently bonded transition metal and dichalcogenide atoms arranged in a hexagonal lattice where adjacent layers are held together by relatively weak van der Waals forces. MoS2 is one such example of TMDs that exhibits a direct band gap of 1.8 eV for single-layer MoS2 and an indirect band gap of 1.2 eV for multiple-layer structure. FETs based on single-layer MoS2 exhibit an on/off ratio of > 108 at room temperature. However, unlike graphene-based FETs which typically exhibit a high mobility of 5,000 - 10,000 cm2V-1s-1, MoS2-based FETs show a much lower mobility of ~100 -200 cm2V-1s-1.

One possible way of tailoring the electrical properties of a material is by stacking individual layers of different 2D materials to form a vertical heterostructure. In this dissertation, we explored the electrical properties of graphene - MoS2 hybrid films synthesized by direct growth of single-layer and multi-layer MoS2 on graphene using chemical vapor deposition (CVD). By taking advantage of the high charge carrier mobility of graphene and the finite band gap of MoS2, we fabricate FETs which consist of graphene - MoS2 hybrid as the conducting channel. We explore the potential of this new hybrid structure for electro-chemical sensing by testing its selectivity and sensitivity for different volatile organic compounds (VOCs). In addition, we study an optoelectronic sensor using graphene as electrodes and MoS2 as conducting/sensing channel. The red-light with matching energy to band gap in single-layer MoS2 increases electron concentration in MoS2 and hence promotes the current in the sensor. The optoelectronic sensor was used for NO2 detection. In addition, another heterostructure of MoS2 and metal oxide was also investigated for DNA sensor. Finally, we studied the potentials of MoS2 and graphene heterostructure for a potential flexible and on-skin applications for NO2 and NO detection.

    Cover page: MoS2 Based Heterostructures for Enhanced Chemical and Biological Sensing
    • Article
    • Peer Reviewed
    UC Riverside Previously Published Works (2018)
    Improvement upon, and expansion of, diagnostic tools for clinical infections have been increasing in recent years. The simplicity and rapidity of techniques are imperative for their adoption and widespread usage at point-of-care. The fabrication and evaluation of such a device is reported in this work. The use of a small bioreceptor array (based on lectin-carbohydrate binding) resulted in a unique response profile, which has the potential to be used for pathogen identification, as demonstrated by Principal Component Analysis (PCA). The performance of the chemiresistive device was tested with Escherichia coli K12, Enterococcus faecalis, Streptococcus mutans, and Salmonella typhi. The limits of detection, based on concanavalin A (conA) lectin as the bioreceptor, are 4.7 × 10³ cfu/mL, 25 cfu/mL, 7.4 × 10⁴ cfu/mL, and 6.3 × 10² cfu/mL. This shows that the detection of pathogenic bacteria is achieved with clinically relevant concentrations. Importantly, responses measured in spiked artificial saliva showed minimal matrix interference. Furthermore, the exploitation of the distinctive outer composition of the bacteria and selectivity of lectin-carbohydrate interactions allowed for the discrimination of bacterial infections from viral infections, which is a current and urgent need for diagnosing common clinical infections.
      Cover page: Lectin- and Saccharide-Functionalized Nano-Chemiresistor Arrays for Detection and Identification of Pathogenic Bacteria Infection
      • Article
      • Peer Reviewed
      UC Riverside Previously Published Works (2019)
      Volatile organic compounds (VOCs) released by plants are closely associated with plant metabolism and can serve as biomarkers for disease diagnosis. Huanglongbing (HLB), also known as citrus greening or yellow shoot disease, is a lethal threat to the multi-billion-dollar citrus industry. Early detection of HLB is vital for removal of susceptible citrus trees and containment of the disease. Gas sensors are applied to monitor the air quality or toxic gases owing to their low-cost fabrication, smooth operation, and possible miniaturization. Here, we report on the development, characterization, and application of electrical biosensor arrays based on single-walled carbon nanotubes (SWNTs) decorated with single-stranded DNA (ssDNA) for the detection of four VOCs-ethylhexanol, linalool, tetradecene, and phenylacetaldehyde-that serve as secondary biomarkers for detection of infected citrus trees during the asymptomatic stage. SWNTs were noncovalently functionalized with ssDNA using π-π interaction between the nucleotide and sidewall of SWNTs. The resulting ssDNA-SWNT hybrid structure and device properties were investigated using Raman spectroscopy, ultraviolet (UV) spectroscopy, and electrical measurements. To monitor changes in the four VOCs, gas biosensor arrays consisting of bare SWNTs before and after being decorated with different ssDNA were employed to determine the different concentrations of the four VOCs. The data was processed using principal component analysis (PCA) and neural net fitting (NNF).
        Cover page: Gas Biosensor Arrays Based on Single-Stranded DNA-Functionalized Single-Walled Carbon Nanotubes for the Detection of Volatile Organic Compound Biomarkers Released by Huanglongbing Disease-Infected Citrus Trees
        • Article
        • Peer Reviewed
        UC Riverside Previously Published Works (2021)
        Molybdenum disulfide (MoS2) features a band gap of 1.3 eV (indirect) to 1.9 eV (direct). This tunable band gap renders MoS2 a suitable conducting channel for field-effect transistors (FETs). In addition, the highly sensitive surface potential in MoS2 layers allows the feasibility of FET applications in biosensors, where direct immobilization and detection of biological molecules are conducted in wet conditions. In this work, we report, for the first time, the degradation of chemical vapor deposition (CVD) grown MoS2 FET-based sensors in the presence of phosphate buffer and water, which caused false positive response in detection. We conclude the degradation was originated by physical delamination of MoS2 thin films from the SiO2 substrate. The problem was alleviated by coating the sensors with a 30 nm thick aluminum oxide (Al2O3) layer using atomic layer deposition technique (ALD). This passive oxide thin film not only acted as a protecting layer against the device degradation but also induced a strong n-doping onto MoS2, which permitted a facile method of detection in MoS2 FET-based sensors using a low-power mode chemiresistive I-V measurement at zero gate voltage (Vgate = 0 V). Additionally, the oxide layer provided available sites for facile functionalization with bioreceptors. As immunoreaction plays a key role in clinical diagnosis and environmental analysis, our work presented a promising application using such enhanced Al2O3-coated MoS2 chemiresistive biosensors for detection of HIgG with high sensitivity and selectivity. The biosensor was successfully applied to detect HIgG in artificial urine, a complex matrix containing organics and salts.
          Cover page: Effect of Al2O3 Passive Layer on Stability and Doping of MoS2 Field-Effect Transistor (FET) Biosensors
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