Abdullah Alharbi

Electronic properties of monolayer tungsten disulfide grown by chemical vapor deposition

We demonstrate chemical vapor deposition of large monolayer tungsten disulfide (WS2) (>200 μm). Photoluminescence and Raman spectroscopy provide insight into the structural and strain heterogeneity of the flakes. We observe exciton quenching at grain boundaries that originate from the nucleation site at the center of the WS2 flakes. Temperature variable transport measurements of top-gated WS2 transistors show an apparent metal-to-insulator transition. Variable range and thermally activated hopping mechanisms can explain the carrier transport in the insulating phase at low and intermediate temperatures. The devices exhibit room-temperature field-effect electron mobility as high as 48 cm2/V.s. The mobility increases with decreasing temperature and begins to saturate at below 100 °K, possibly due to Coulomb scattering or defects.

Material and device properties of superacid-treated monolayer molybdenum disulfide

We study the effects of chemical treatment with bis(trifluoromethane) sulfonimide superacid on material and device properties of monolayer molybdenum disulfide grown by chemical vapor deposition. Our spatially resolved photoluminescence (PL) measurements and device studies reveal two key findings due to the chemical treatment: (1) noticeable transformation of trions to neutral excitons, and (2) over 7-fold reduction in the density of mid-gap trap states. Specifically, a combination of scanning Auger microscopy and PL mapping reveals that the superacid treatment is effective in passivating the sulfur-deficient regions.

15.7 Heterogeneous integrated CMOS-graphene sensor array for dopamine detection

Understanding dopamine (DA) signaling in the brain is essential for advancing our knowledge of pathological disorders such as drug addiction, Parkinsons disease, and schizophrenia. Currently, fast-scan cyclic voltammetry (FSCV) with carbon microfiber (CMF) electrodes is the method of choice in neuroscience labs for monitoring the concentration of phasic (transient) DA release. This method offers sub-second temporal resolution and high specificity because the signal of interest occurs at a known potential. However, existing CMF electrodes are bulky, limiting the spatial resolution to single-site measurements. Further, they are produced through manual processes (e.g. cutting CMFs under optical microscope), thus introducing significant device variability [1]. Lastly, when long probes (3-to-5cm) are used to monitor DA release in deep brain structures of large animals, environmental noise severely diminishes the detection limit [1]. To address these problems, we combine advances in nanofabrication with silicon chip manufacturing to create a heterogeneous integrated CMOS-graphene sensor for accurate measurement of DA with high spatiotemporal resolution (Fig. 15.7.1).

Advanced integrated sensor and layer transfer technologies for wearable bioelectronics

We discuss two emerging technologies that are central for realizing an optically powered flexible bioelectronic system. First, we discuss layer transfer through controlled spalling technology for producing high-performance flexible electronics. We present three examples: (1) advanced-node ultra-thin body silicon integrated circuits on plastic, (2) strain engineering in flexible electronics, and (3) flexible GaAs photovoltaic energy harvesters. Second, a 4-terminal biosensor is presented that is compatible with ultra-thin body silicon CMOS technology. Through in vitro glucose sensing, we demonstrate that the 4-terminal integrated biosensor enables the amplification of biochemical signals at the device level. These advanced technologies can give rise to an unprecedented boost in the performance and functionality of next-generation wearable devices.

Contact engineering of monolayer CVD MOS 2 transistors

Transition metal dichalcogenides (TMDs) are promising for next-generation electronic and optoelectronic device applications. However, the development of a viable TMD device technology requires an effective strategy for making low-resistance contacts to these materials. In addition, large-area synthesis of low-defect TMD crystals is essential for transforming basic device studies into commercial products. Here, we show large-area synthesis of monolayer (ML) MoS<inf>2</inf> using chemical vapor deposition (CVD) with electron mobility of ∼64cm²/V.s at room temperature. We performed contact engineering through a combination of work function engineering and effective n-type doping of MoS<inf>2</inf> using engineered sub-stoichiometric HfO<inf>x</inf>. Our results indicate significant reduction of the contact resistance to ∼480Ω.μm without degrading key transistor properties such as subthreshold swing (∼125mV/dec), mobility (∼64cm²/V.s), and I<inf>ON</inf>/I<inf>OFF</inf> ratio (>10⁶).

Optical identification using imperfections in 2D materials

The ability to uniquely identify an object or device is important for authentication. Imperfections, locked into structures during fabrication, can be used to provide a fingerprint that is challenging to reproduce. In this paper, we propose a simple optical technique to read unique information from nanometer-scale defects in 2D materials. Flaws created during crystal growth or fabrication lead to spatial variations in the bandgap of 2D materials that can be characterized through photoluminescence measurements. We show a simple setup involving an angle-adjustable transmission filter, simple optics and a CCD camera can capture spatially-dependent photoluminescence to produce complex maps of unique information from 2D monolayers. Atomic force microscopy is used to verify the origin of the optical signature measured, demonstrating that it results from nanometer-scale imperfections. This solution to optical identification with 2D materials could be employed as a robust security measure to prevent counterfeiting.

Physically Unclonable Cryptographic Primitives by Chemical Vapor Deposition of Layered MoS2

Physically unclonable cryptographic primitives are promising for securing the rapidly growing number of electronic devices. Here, we introduce physically unclonable primitives from layered molybdenum disulfide (MoS2) by leveraging the natural randomness of their island growth during chemical vapor deposition (CVD). We synthesize a MoS2 monolayer film covered with speckles of multilayer islands, where the growth process is engineered for an optimal speckle density. Using the Clark-Evans test, we confirm that the distribution of islands on the film exhibits complete spatial randomness, hence indicating the growth of multilayer speckles is a spatial Poisson process. Such a property is highly desirable for constructing unpredictable cryptographic primitives. The security primitive is an array of 2048 pixels fabricated from this film. The complex structure of the pixels makes the physical duplication of the array impossible (i.e., physically unclonable). A unique optical response is generated by applying an optical stimulus to the structure. The basis for this unique response is the dependence of the photoemission on the number of MoS2 layers, which by design is random throughout the film. Using a threshold value for the photoemission, we convert the optical response into binary cryptographic keys. We show that the proper selection of this threshold is crucial for maximizing combination randomness and that the optimal value of the threshold is linked directly to the growth process. This study reveals an opportunity for generating robust and versatile security primitives from layered transition metal dichalcogenides.

Experimental Study of the Detection Limit in Dual-Gate Biosensors Using Ultrathin Silicon Transistors

Dual-gate field-effect biosensors (bioFETs) with asymmetric gate capacitances were shown to surpass the Nernst limit of 59 mV/pH. However, previous studies have conflicting findings on the effect of the capacitive amplification scheme on the sensor detection limit, which is inversely proportional to the signal-to-noise ratio (SNR). Here, we present a systematic experimental investigation of the SNR using ultrathin silicon transistors. Our sensors operate at low voltage and feature asymmetric front and back oxide capacitances with asymmetry factors of 1.4 and 2.3. We demonstrate that in the dual-gate configuration, the response of our bioFETs to the pH change increases proportional to the asymmetry factor and indeed exceeds the Nernst limit. Further, our results reveal that the noise amplitude also increases in proportion to the asymmetry factor. We establish that the commensurate increase of the noise amplitude originates from the intrinsic low-frequency characteristic of the sensor noise, dominated by number fluctuation. These findings suggest that this capacitive signal amplification scheme does not improve the intrinsic detection limit of the dual-gate biosensors.

Hybrid CMOS-Graphene Sensor Array for Subsecond Dopamine Detection

We introduce a hybrid CMOS-graphene sensor array for subsecond measurement of dopamine via fast-scan cyclic voltammetry (FSCV). The prototype chip has four independent CMOS readout channels, fabricated in a 65-nm process. Using planar multilayer graphene as biologically compatible sensing material enables integration of miniaturized sensing electrodes directly above the readout channels. Taking advantage of the chemical specificity of FSCV, we introduce a region of interest technique, which subtracts a large portion of the background current using a programmable low-noise constant current at about the redox potentials. We demonstrate the utility of this feature for enhancing the sensitivity by measuring the sensor response to a known dopamine concentration in vitro at three different scan rates. This strategy further allows us to significantly reduce the dynamic range requirements of the analog-to-digital converter (ADC) without compromising the measurement accuracy. We show that an integrating dual-slope ADC is adequate for digitizing the background-subtracted current. The ADC operates at a sampling frequency of 5–10 kHz and has an effective resolution of about 60 pA, which corresponds to a theoretical dopamine detection limit of about 6 nM. Our hybrid sensing platform offers an effective solution for implementing next-generation FSCV devices that can enable precise recording of dopamine signaling in vivo on a large scale.

Security engineering of nanostructures and nanomaterials

Proliferation of electronics and their increasing connectivity pose formidable challenges for information security. At the most fundamental level, nanostructures and nanomaterials offer an unprecedented opportunity to introduce new approaches to securing electronic devices. First, we discuss engineering nanomaterials, (e.g., carbon nanotubes (CNTs), graphene, and layered transition metal dichalcogenides (TMDs)) to make unclonable cryptographic primitives. These security primitives not only can supplement existing solutions in silicon integrated circuits (ICs) but can also be used for emerging applications in flexible and wearable electronics. Second, we discuss security engineering of advanced nanostructures such as reactive materials.