Toward robust quantification of dopamine and serotonin in mixtures using nano-graphitic carbon sensors.

Monitoring the coordinated signaling of dopamine (DA) and serotonin (5-HT) is important for advancing our understanding of the brain. However, the co-detection and robust quantification of these signals at low concentrations is yet to be demonstrated. Here, we present the quantification of DA and 5-HT using nano-graphitic (NG) sensors together with fast-scan cyclic voltammetry (FSCV) employing an engineered N-shape potential waveform. Our method yields 6% error in quantifying DA and 5-HT analytes present in in vitro mixtures at concentrations below 100 nM. This advance is due to the electrochemical properties of NG sensors which, in combination with the engineered FSCV waveform, provided distinguishable cyclic voltammograms (CVs) for DA and 5-HT. We also demonstrate the generalizability of the prediction model across different NG sensors, which arises from the consistent voltammetric fingerprints produced by our NG sensors. Curiously, the proposed engineered waveform also improves the distinguishability of DA and 5-HT CVs obtained from traditional carbon fiber (CF) microelectrodes. Nevertheless, this improved distinguishability of CVs obtained from CF is inferior to that of NG sensors, arising from differences in the electrochemical properties of the sensor materials. Our findings demonstrate the potential of NG sensors and our proposed FSCV waveform for future brain studies.

Mechanisms of Interface Cleaning in Heterostructures Made from Polymer-Contaminated Graphene.

Heterostructures obtained from layered assembly of 2D materials such as graphene and hexagonal boron nitride have potential in the development of new electronic devices. Whereas various materials techniques can now produce macroscopic scale graphene, the construction of similar size heterostructures with atomically clean interfaces is still unrealized. A primary barrier has been the inability to remove polymeric residues from the interfaces that arise between layers when fabricating heterostructures. Here, the interface cleaning problem of polymer-contaminated heterostructures is experimentally studied from an energy viewpoint. With this approach, it is established that the interface cleaning mechanism involves a combination of thermally activated polymer residue mobilization and their mechanical actuation. This framework allows a systematic approach for fabricating record large-area clean heterostructures from polymer-contaminated graphene. These heterostructures provide state-of-the-art electronic performance. This study opens new strategies for the scalable production of layered materials heterostructures.

Unraveling the complex electrochemistry of serotonin using engineered graphitic sensors.

Fast-scan cyclic voltammetry (FSCV) with micron-sized carbon sensors is a promising approach for monitoring the fast dynamics of serotonin (5-HT) neuromodulatory signals in the brain. However, sensor performance using FSCV can be compromised by complex chemical reactions associated with the reduction and oxidation of 5-HT, posing considerable challenges to detection of 5-HT in vivo. Herein we describe the use of engineered graphitic sensors to characterize the complex electrochemistry of 5-HT under a wide range of measurement conditions, with the aim of optimizing the FSCV conditions for in vivo quantitative 5-HT detection. These measurements reveal that water plays a significant role in driving side reactions during low-voltage FSCV measurements, leading to the observation of a well-defined secondary redox couple we associated with the redox reaction of tryptamine 4,5-dione. Remarkably, these side reactions can persist subsequent to the primary redox events associated with 5-HT. Furthermore, the results reveal a critical deviation from this ideal redox behavior if the FSCV anodic limit exceeds +0.8 V, which can be attributed to the generation of radical species from water oxidation. These new insights could lead to new FSCV protocols for more reliable 5-HT detection.

Extremely flexible nanoscale ultrathin body silicon integrated circuits on plastic.

In recent years, flexible devices based on nanoscale materials and structures have begun to emerge, exploiting semiconductor nanowires, graphene, and carbon nanotubes. This is primarily to circumvent the existing shortcomings of the conventional flexible electronics based on organic and amorphous semiconductors. The aim of this new class of flexible nanoelectronics is to attain high-performance devices with increased packing density. However, highly integrated flexible circuits with nanoscale transistors have not yet been demonstrated. Here, we show nanoscale flexible circuits on 60 Å thick silicon, including functional ring oscillators and memory cells. The 100-stage ring oscillators exhibit the stage delay of ~16 ps at a power supply voltage of 0.9 V, the best reported for any flexible circuits to date. The mechanical flexibility is achieved by employing the controlled spalling technology, enabling the large-area transfer of the ultrathin body silicon devices to a plastic substrate at room temperature. These results provide a simple and cost-effective pathway to enable ultralight flexible nanoelectronics with unprecedented level of system complexity based on mainstream silicon technology.

Anomalous sensitivity enhancement of nano-graphitic electrochemical micro-sensors with reducing the operating voltage.

Microscopic interactions between electrochemical sensors and biomolecules critically influence the sensitivity. Here, we report an unexpected dependence of the sensitivity on the upper potential limit (UPL) in voltammetry experiments. In particular, we find that the sensitivity of substrate-supported nano-graphitic micro-sensors in response to dopamine increases almost linearly with the inverse of UPL in voltammetry experiments with rapid potential sweeps. Our experiments and multi-physics simulations reveal that the main cause behind this phenomenon is the UPL-induced electrostatic force that influences the steady-state number of dopamine molecules on the sensor surface. Our findings illustrate a new strategy for enhancing the performance of planar electrochemical micro-sensors.

An Electrochemical Biochip for Measuring Low Concentrations of Analytes With Adjustable Temporal Resolutions.

Electrochemical micro-sensors made of nano-graphitic (NG) carbon materials could offer high sensitivity and support voltammetry measurements at vastly different temporal resolutions. Here, we implement a configurable CMOS biochip for measuring low concentrations of bio-analytes by leveraging these advantageous features of NG micro-sensors. In particular, the core of the biochip is a discrete-time ∆Σ modulator, which can be configured for optimal power consumption according to the temporal resolution requirements of the sensing experiments while providing a required precision of ≈ 13 effective number of bits. We achieve this new functionality by developing a design methodology using the physical models of transistors, which allows the operating region of the modulator to be switched on-demand between weak and strong inversion. We show the application of this configurable biochip through in-vitro measurements of dopamine with concentrations as low as 50 nM and 200 nM at temporal resolutions of 100 ms and 10 s, respectively.

Versatile construction of van der Waals heterostructures using a dual-function polymeric film.

The proliferation of van der Waals (vdW) heterostructures formed by stacking layered materials can accelerate scientific and technological advances. Here, we report a strategy for constructing vdW heterostructures through the interface engineering of the exfoliation substrate using a sub-5 nm polymeric film. Our construction method has two main features that distinguish it from existing techniques. First is the consistency of its exfoliation process in increasing the yield and in producing large (>10,000 µm2) monolayer graphene. Second is the applicability of its layer transfer process to different layered materials without requiring a specialized stamp-a feature useful for generalizing the assembly process. We demonstrate vdW graphene devices with peak carrier mobility of 200,000 and 800,000 cm2 V-1 s-1 at room temperature and 9 K, respectively. The simplicity of our construction method and its versatility to different layered materials may open doors for automating the fabrication process of vdW heterostructures.

Nano-engineering the material structure of preferentially oriented nano-graphitic carbon for making high-performance electrochemical micro-sensors.

Direct synthesis of thin-film carbon nanomaterials on oxide-coated silicon substrates provides a viable pathway for building a dense array of miniaturized (micron-scale) electrochemical sensors with high performance. However, material synthesis generally involves many parameters, making material engineering based on trial and error highly inefficient. Here, we report a two-pronged strategy for producing engineered thin-film carbon nanomaterials that have a nano-graphitic structure. First, we introduce a variant of the metal-induced graphitization technique that generates micron-scale islands of nano-graphitic carbon materials directly on oxide-coated silicon substrates. A novel feature of our material synthesis is that, through substrate engineering, the orientation of graphitic planes within the film aligns preferentially with the silicon substrate. This feature allows us to use the Raman spectroscopy for quantifying structural properties of the sensor surface, where the electrochemical processes occur. Second, we find phenomenological models for predicting the amplitudes of the redox current and the sensor capacitance from the material structure, quantified by Raman. Our results indicate that the key to achieving high-performance micro-sensors from nano-graphitic carbon is to increase both the density of point defects and the size of the graphitic crystallites. Our study offers a viable strategy for building planar electrochemical micro-sensors with high-performance.

Spatial defects nanoengineering for bipolar conductivity in MoS2.

Understanding the atomistic origin of defects in two-dimensional transition metal dichalcogenides, their impact on the electronic properties, and how to control them is critical for future electronics and optoelectronics. Here, we demonstrate the integration of thermochemical scanning probe lithography (tc-SPL) with a flow-through reactive gas cell to achieve nanoscale control of defects in monolayer MoS2. The tc-SPL produced defects can present either p- or n-type doping on demand, depending on the used gasses, allowing the realization of field effect transistors, and p-n junctions with precise sub-µm spatial control, and a rectification ratio of over 104. Doping and defects formation are elucidated by means of X-Ray photoelectron spectroscopy, scanning transmission electron microscopy, and density functional theory. We find that p-type doping in HCl/H2O atmosphere is related to the rearrangement of sulfur atoms, and the formation of protruding covalent S-S bonds on the surface. Alternatively, local heating MoS2 in N2 produces n-character.

Quantitative Principles for Precise Engineering of Sensitivity in Graphene Electrochemical Sensors.

A major difficulty in implementing carbon-based electrode arrays with high device-packing density is to ensure homogeneous and high sensitivities across the array. Overcoming this obstacle requires quantitative microscopic models that can accurately predict electrode sensitivity from its material structure. Such models are currently lacking. Here, it is shown that the sensitivity of graphene electrodes to dopamine and serotonin neurochemicals in fast-scan cyclic voltammetry measurements is strongly linked to point defects, whereas it is unaffected by line defects. Using the physics of point defects in graphene, a microscopic model is introduced that explains how point defects determine sensitivity. The predictions of this model match the empirical observation that sensitivity linearly increases with the density of point defects. This model is used to guide the nanoengineering of graphene structures for optimum sensitivity. This approach achieves reproducible fabrication of miniaturized sensors with extraordinarily higher sensitivity than conventional materials. These results lay the foundation for new integrated electrochemical sensor arrays based on nanoengineered graphene.

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.

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.

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.

High-performance air-stable n-type carbon nanotube transistors with erbium contacts.

So far, realization of reproducible n-type carbon nanotube (CNT) transistors suitable for integrated digital applications has been a difficult task. In this work, hundreds of n-type CNT transistors from three different low work function metals-erbium, lanthanum, and yttrium-are studied and benchmarked against p-type devices with palladium contacts. The crucial role of metal type and deposition conditions is elucidated with respect to overall yield and performance of the n-type devices. It is found that high oxidation rates and sensitivity to deposition conditions are the major causes for the lower yield and large variation in performance of n-type CNT devices with low work function metal contacts. Considerable improvement in device yield is attained using erbium contacts evaporated at high deposition rates. Furthermore, the air-stability of our n-type transistors is studied in light of the extreme sensitivity of these metals to oxidation.

Variability in carbon nanotube transistors: improving device-to-device consistency.

The large amount of hysteresis and threshold voltage variation in carbon nanotube transistors impedes their use in highly integrated digital applications. The origin of this variability is elucidated by employing a top-coated, hydrophobic monolayer to passivate bottom-gated devices. Compared to passivating only the supporting substrate, it is found that covering the nanotube channel proves highly effective and robust at improving device-to-device consistency-hysteresis and threshold voltage variation are reduced by an average of 84 and 53%, respectively. The effect of gate and drain-source bias on hysteresis is considered, showing strong dependence that must be accounted for when analyzing the effectiveness of a passivation layer. These results provide both key insight into the origin of variability in carbon nanotube transistors and a promising path for resolving this significant obstacle.