Disorder-tuned conductivity in amorphous monolayer carbon.

Correlating atomic configurations-specifically, degree of disorder (DOD)-of an amorphous solid with properties is a long-standing riddle in materials science and condensed matter physics, owing to difficulties in determining precise atomic positions in 3D structures1-5. To this end, 2D systems provide insight to the puzzle by allowing straightforward imaging of all atoms6,7. Direct imaging of amorphous monolayer carbon (AMC) grown by laser-assisted depositions has resolved atomic configurations, supporting the modern crystallite view of vitreous solids over random network theory8. Nevertheless, a causal link between atomic-scale structures and macroscopic properties remains elusive. Here we report facile tuning of DOD and electrical conductivity in AMC films by varying growth temperatures. Specifically, the pyrolysis threshold temperature is the key to growing variable-range-hopping conductive AMC with medium-range order (MRO), whereas increasing the temperature by 25 °C results in AMC losing MRO and becoming electrically insulating, with an increase in sheet resistance of 109 times. Beyond visualizing highly distorted nanocrystallites embedded in a continuous random network, atomic-resolution electron microscopy shows the absence/presence of MRO and temperature-dependent densities of nanocrystallites, two order parameters proposed to fully describe DOD. Numerical calculations establish the conductivity diagram as a function of these two parameters, directly linking microstructures to electrical properties. Our work represents an important step towards understanding the structure-property relationship of amorphous materials at the fundamental level and paves the way to electronic devices using 2D amorphous materials.

Re-understanding of SERS for General and Standardized Quantitative Analysis.

We proposed two physical concepts, i.e., an intramolecular relative cross section (RCS) and an intermolecular relative scattering ability (RSA), to re-understand and re-describe surface-enhanced Raman scattering (SERS) and established a general SERS quantification theory. Interestingly, RCS and RSA are intrinsic factors and are experimentally measurable to form datasheets of molecules, namely, SERS cards, with which a standard SERS quantification procedure was established. The validity of the theory and quantification procedure was confirmed by experiments. Surprisingly, RCS and RSA are also valid for complex systems being considered as virtual molecules and are experimentally measurable. This simplifies complex systems into analyte-virtual molecule binary systems. With this consideration, trace-level mitoxantrone (a typical cancer drug metabolite) in artificial urine was accurately predicted. The theory, the SERS cards, the standard quantification procedure, and the virtual molecule concept pave a way toward quantitative and standardized SERS spectroscopy in dealing with real-world problems and complex samples.

Joint Exfoliation of MXene by Dimensional Mismatched SiC/ZIF-67 Toward Multifunctional Flame Retardant Thermoplastic Polyurethane.

The single-layer MXene fully demonstrates the advantages of 2D materials, especially catalytic, conductive, and mechanical properties. However, the high energy consumption and low efficiency faced by MXene in the divestiture process are still challenges that need to be solved urgently. In this article, dimension mismatch and collaborative stripping strategies are skillfully combined to easily realize the transformation from multi-layer MXene to single layer. In addition, the functionalized MXene@SiC@polyaniline (MXene@SiC@PANI) nano-hybrid materials are used as fillers to improve the thermal conductivity, flame retardant, and antibacterial properties of thermoplastic polyurethane (TPU). The surface temperature of TPU/MXene@SiC@PANI composites increased from 33.4 °C to 59.8 °C within 10 s. In addition, the antibacterial efficiency of TPU composites against Escherichia coli and Staphylococcus aureus is 69.6% and 88.9%, respectively. Compared with pure TPU, the peak heat release rate and total heat release are reduced by 71.4% and 34.6%, respectively. The flame-retardant mechanism of MXene hybrid materials is systematically discussed. It is worth noting that the introduction of PANI enhances the compatibility between the filler and the polymer, effectively maintaining the mechanical properties of the TPU itself. This work provides a convenient method for the multi-functional practical application of TPU.

Ultrasensitive Biochemical Sensing Platform Enabled by Directly Grown Graphene on Insulator.

To fabricate label-free and rapid-resulting semiconducting biosensor devices incorporating graphene, it is pertinent to directly grow uniform graphene films on technologically important dielectric and semiconducting substrates. However, it has long been intuitively believed that the nonideal disordered structures formed during direct growth, and the resulted inferior electrical properties will inevitably lead to deteriorated sensing performance. Here, graphene biosensor chips are constructed based on direct plasma-enhanced chemical vapor deposition (PECVD) grown graphene on a 4-inch silicon wafer with excellent film uniformity and high yield. To surprise, optimal operations of graphene biosensors permit ultrasensitive detection of SARS-CoV-2 virus nucleocapsid protein with dilutions down to sub-femtomolar concentrations. Such impressive limit of detection (LOD) is comparable to or even outperforms that of the state-of-the-art biosensor devices based on high-quality graphene. Further noise spectral characterizations and analysis confirms that the LOD is limited by molecular diffusion and/or known interference signals such as drift and instability of the sensors, rather than the electrical merits of the graphene devices along. Hence, result sheds light on processing directly grown PECVD graphene into high-performance sensor devices with important economic benefits and social significance.

Hybrid superlattices of two-dimensional materials and organics.

Two-dimensional (2D) materials have received extensive interest due to their exceptional properties. It is strongly required to assemble 2D materials in bulk quantities for macroscopic applications, but this is highly restricted by the aggregation of 2D materials. Constructing three-dimensional (3D) hybrid superlattices of alternating 2D materials and organic molecule layers provides a new path to access the exceptional properties of 2D materials in bulk quantities. In this tutorial review, the emerging concept of hybrid inorganic/organic superlattices is systematically illustrated. The abundant compositions and the various structures of inorganic and organic sublattices in hybrid superlattices are presented, followed by a summary of the chemical interactions between them. Many facile techniques have been developed for hybrid superlattices, enabling precise control of the structure. There are also various interesting mechanisms inside unique hybrid inorganic/organic superlattices that can help tune the properties, including electron transfer, quantum confinement, interlayer coupling, multiple interface effects, etc. The rich chemistry and abundant mechanisms of these hybrid superlattices can enhance the performance beyond the reach of existing materials, and provide new opportunities in various applications, including rechargeable batteries, catalysis, thermoelectrics, advanced electronics, superconductors, optoelectronics, etc.

Ultrasensitive Field-Effect Biosensors Enabled by the Unique Electronic Properties of Graphene.

This review provides a critical overview of current developments on nanoelectronic biochemical sensors based on graphene. Composed of a single layer of conjugated carbon atoms, graphene has outstanding high carrier mobility and low intrinsic electrical noise, but a chemically inert surface. Surface functionalization is therefore crucial to unravel graphene sensitivity and selectivity for the detection of targeted analytes. To achieve optimal performance of graphene transistors for biochemical sensing, the tuning of the graphene surface properties via surface functionalization and passivation is highlighted, as well as the tuning of its electrical operation by utilizing multifrequency ambipolar configuration and a high frequency measurement scheme to overcome the Debye screening to achieve low noise and highly sensitive detection. Potential applications and prospectives of ultrasensitive graphene electronic biochemical sensors ranging from environmental monitoring and food safety, healthcare and medical diagnosis, to life science research, are presented as well.

Ultrasensitive Ethene Detector Based on a Graphene-Copper(I) Hybrid Material.

Ethene is a highly diffusive and relatively unreactive gas that induces aging responses in plants in concentrations as low as parts per billion. Monitoring concentrations of ethene is critically important for transport and storage of food crops, necessitating the development of a new generation of ultrasensitive detectors. Here we show that by functionalizing graphene with copper complexes biologically relevant concentrations of ethene and of the spoilage marker ethanol can be detected. Importantly, in addition these sensors provide us with important insights into the interactions between molecules, a key concept in chemistry. Chemically induced dipole fluctuations in molecules as they undergo a chemical reaction are harvested in an elegant way through subtle field effects in graphene. By exploiting changes in the dipole moments of molecules that occur upon a chemical reaction we are able to track the reaction and provide mechanistic insight that was, until now, out of reach.

Electrolyte-Gated Graphene Ambipolar Frequency Multipliers for Biochemical Sensing.

In this Letter, the ambipolar properties of an electrolyte-gated graphene field-effect transistor (GFET) have been explored to fabricate frequency-doubling biochemical sensor devices. By biasing the ambipolar GFETs in a common-source configuration, an input sinusoidal voltage at frequency f applied to the electrolyte gate can be rectified to a sinusoidal wave at frequency 2f at the drain electrode. The extraordinary high carrier mobility of graphene and the strong electrolyte gate coupling provide the graphene ambipolar frequency doubler an unprecedented unity gain, as well as a detection limit of ∼4 pM for 11-mer single strand DNA molecules in 1 mM PBS buffer solution. Combined with an improved drift characteristics and an enhanced low-frequency 1/f noise performance by sampling at doubled frequency, this good detection limit suggests the graphene ambipolar frequency doubler a highly promising biochemical sensing platform.

Low-bias active control of terahertz waves by coupling large-area CVD graphene to a terahertz metamaterial.

We propose an hybrid graphene/metamaterial device based on terahertz electronic split-ring resonators directly evaporated on top of a large-area single-layer CVD graphene. Room temperature time-domain spectroscopy measurements in the frequency range from 250 GHz to 2.75 THz show that the presence of the graphene strongly changes the THz metamaterial transmittance on the whole frequency range. The graphene gating allows active control of such interaction, showing a modulation depth of 11.5% with an applied bias of 10.6 V. Analytical modeling of the device provides a very good qualitative and quantitative agreement with the measured device behavior. The presented system shows potential as a THz modulator and can be relevant for strong light-matter coupling experiments.

Degree of disorder-regulated ion transport through amorphous monolayer carbon.

Nanopore technology, re-fueled by two-dimensional (2D) materials such as graphene and MoS2, controls mass transport by allowing certain species while denying others at the nanoscale and has a wide application range in DNA sequencing, nano-power generation, and others. With their low transmembrane transport resistance and high permeability stemming from their ultrathin nature, crystalline 2D materials do not possess nanoscale holes naturally, thus requiring additional fabrication to create nanopores. Herein, we demonstrate that nanopores exist in amorphous monolayer carbon (AMC) grown at low temperatures. The size and density of nanopores can be tuned by the growth temperature, which was experimentally verified by atomic images and further corroborated by kinetic Monte Carlo simulation. Furthermore, AMC films with varied degrees of disorder (DOD) exhibit tunable transmembrane ionic conductance over two orders of magnitude when serving as nanopore membranes. This work demonstrates the DOD-tuned property in amorphous monolayer carbon and provides a new candidate for modern membrane science and technology.

Recognition of dipole-induced electric field in 2D materials for surface-enhanced Raman scattering.

The application of two-dimensional (2D) materials, including metallic graphene, semiconducting transition metal dichalcogenides, and insulating hexagonal boron nitride (h-BN) for surface-enhancement Raman spectroscopy has attracted extensive research interest. This article provides a critical overview of the recent developments in surface-enhanced Raman spectroscopy using 2D materials. By re-examining the relationship between the lattice structure and Raman enhancement characteristics, including vibration selectivity and thickness dependence, we highlight the important role of dipoles in the chemical enhancement of 2D materials.

Graphene transistors are insensitive to pH changes in solution.

We observe very small gate-voltage shifts in the transfer characteristic of as-prepared graphene field-effect transistors (GFETs) when the pH of the buffer is changed. This observation is in strong contrast to Si-based ion-sensitive FETs. The low gate-shift of a GFET can be further reduced if the graphene surface is covered with a hydrophobic fluorobenzene layer. If a thin Al-oxide layer is applied instead, the opposite happens. This suggests that clean graphene does not sense the chemical potential of protons. A GFET can therefore be used as a reference electrode in an aqueous electrolyte. Our finding sheds light on the large variety of pH-induced gate shifts that have been published for GFETs in the recent literature.

Ferroelectric gated electrical transport in CdS nanotetrapods.

Complex nanostructures such as branched semiconductor nanotetrapods are promising building blocks for next-generation nanoelectronics. Here we report on the electrical transport properties of individual CdS tetrapods in a field effect transistor (FET) configuration with a ferroelectric Ba(0.7)Sr(0.3)TiO(3) film as high-k, switchable gate dielectric. A cryogenic four-probe scanning tunneling microscopy (STM) is used to probe the electrical transport through individual nanotetrapods at different temperatures. A p-type field effect is observed at room temperature, owing to the enhanced gate capacitance coupling. And the reversible remnant polarization of the ferroelectric gate dielectric leads to a well-defined nonvolatile memory effect. The field effect is shown to originate from the channel tuning in the arm/core/arm junctions of nanotetrapods. At low temperature (8.5 K), the nanotetrapod devices exhibit a ferroelectric-modulated single-electron transistor (SET) behavior. The results illustrate how the characteristics of a ferroelectric such as switchable polarization and high dielectric constant can be exploited to control the functionality of individual three-dimensional nanoarchitectures.

Dielectric-Modulated Biosensing with Ultrahigh-Frequency-Operated Graphene Field-Effect Transistors.

Owing to their excellent electrical properties and chemical stability, graphene field-effect transistors (Gr-FET) are extensively studied for biosensing applications. However, hinging on surface interactions of charged biomolecules, the sensitivity of Gr-FET is hampered by ionic screening under physiological conditions with high salt concentrations up to frequencies as high as MHz. Here, an electrolyte-gated Gr-FET in reflectometry mode at ultrahigh frequencies (UHF, around 2 GHz), where the ionic screening is fully cancelled and the dielectric sensitivity of the device allows the Gr-FET to directly function in high-salt solutions, is configured. Strikingly, by simultaneous characterization using electrolyte gating and UHF reflectometry, the developed graphene biosensors offer unprecedented capability for real-time monitoring of dielectric-specified biomolecular/cell interactions/activities, with superior limit of detection compared to that of previously reported nanoscale high-frequency sensors. These achievements highlight the unique potential of ultrahigh-frequency operation for unblocking the true potential of graphene biosensors for point-of-care diagnostic.

Tailoring TiO2/Al2O3 heterolayers as optical filters for the visible region.

Optical filters operating in the visible region of the spectrum are highly desired for applications ranging from optical communication and sensing to fluorescence microscopy and skin therapy. However, complex fabrication procedures and/or inferior performance, limit the practical applications of previously reported thin-film-based optical filters. Herein, we describe the structual design concepts and facile fabrication of vertically stacked heterolayers of TiO2/Al2O3 to obtain a bandpass filter and a longwave pass edge filter operating in the spectral range 410-600 nm and 400-597 nm, respectively. The optical filters are designed according to MacLeod simulation and fabricated via magnetron sputtering, depositing alternative stacks of low (Al2O3) and high (TiO2) refractive index materials with different thicknesses, as confirmed by spectroscopic ellipsometry. Owing to a reasonable matching between the design and the fabrication, our developed TiO2/Al2O3 heterolayer optical filters exhibited 54.60% transmission for 7-layer longwave pass edge filter and 15% reflectance for 14-layer bandpass filter of a selective set of wavelengths (400-800 nm).

Cr-Doped Pd Metallene Endows a Practical Formaldehyde Sensor New Limit and High Selectivity.

Electrochemical sensors for detecting micromolecule organics are desirable for improving the perception of environmental quality and human health. However, currently, the electrochemical sensors for formaldehyde are substantially limited on the market due to the long-term unsolved problems of the low electrooxidation efficiency and CO poisoning issue of commercial Pd catalysts. Here, a 2D Cr-doped Pd metallene (Cr-Pdene) with few atomic layers is shown as an advanced catalyst for ultrasensitive and selective sensing of formaldehyde via a highly efficient formaldehyde electrooxidation. It is found that the doping of Cr into Pd metallene can efficiently optimize the electronic structure of Pd and weaken the interaction between Pd and CO, providing an anti-poisoning means to favor CO2 production and suppress CO adsorption. The Cr-Pdene-based electrochemical sensor exhibits one order of magnitude higher detection range and, especially, much higher anti-interference for formaldehyde than that of the conventional sensors. Most importantly, it is demonstrated that the Cr-Pdene can be integrated into commercializable wireless sensor networks or handheld instruments for promising applications relating to the environment, health, and food.

Nernst limit in dual-gated Si-nanowire FET sensors.

Field effect transistors (FETs) are widely used for the label-free detection of analytes in chemical and biological experiments. Here we demonstrate that the apparent sensitivity of a dual-gated silicon nanowire FET to pH can go beyond the Nernst limit of 60 mV/pH at room temperature. This result can be explained by a simple capacitance model including all gates. The consistent and reproducible results build to a great extent on the hysteresis- and leakage-free operation. The dual-gate approach can be used to enhance small signals that are typical for bio- and chemical sensing at the nanoscale.

Robust quantitative SERS analysis with Relative Raman scattering intensities.

Robust quantitative analysis methods are very attractive but challenging with surface-enhanced Raman scattering (SERS) technique till now. Quantitative analysis methods using absolute Raman scattering intensities tend to desire very critical reproducibility of SERS substrates and consistency of testing conditions, as batch differences and inhomogeneity of SERS substrates as well as the fluctuation of measuring parameters placed challenging obstacles. Relative Raman scattering intensities, on the other hand, can release the adverse interferences mentioned above and provide effective and robust information as it is independent of the reproducibility of SERS substrates. By establishing external calibration working curves, we achieved accurate molecule composition prediction of molecules in multi-component systems. Further, by choosing or adding a label molecule with known concentration as Raman internal standards, the concentration of target molecules can be easily predicted. This approach proved the effectiveness and robustness of quantitative analysis with the relative Raman scattering intensities, even carried out with a flexible inhomogeneous SERS substrate.

Investigation of MOF-derived humidity-proof hierarchical porous carbon frameworks as highly-selective toluene absorbents and sensing materials.

Carbon frameworks (CFs) derived from metal-organic frameworks (MOFs) have been produced as adsorbents of toluene. To further obtain optimum hierarchical porous carbon structure of CFs, different treatment temperatures were applied to a typical kind of MOFs (ZIF-8). The adsorption capacity of the toluene of hierarchical porous CFs obtained from ZIF-8 under 1100 °C (CF-1100, adsorption capacity of 208.5 mg/g) was higher than that of other carbonization temperature and MOFs. Impressively, the adsorbent CF-1100 also exhibited strong hydrophobicity, low desorption temperature, and good selectivity to toluene. The adsorption capacity decreased by only 10.4% under wet condition compared with the dry condition, standing on the top of the recently reported adsorbents. The impressive adsorption performance of CF-1100 is attributed to the larger specific surface area (1024 m2/g) and pore volume (0.497 cm3/g), newly generated micropores (pore width is 0.6-0.8 nm) and mesopores (pore width above 10 nm), and carbonaceous structure with higher degree of graphitization. Based on the adequate adsorption performance, CF-1100 coated quartz crystal microbalances as sensor also showed a high sensitivity of 0.4004 Hz/ppm and small relative standard deviations of 1.0745% for toluene sensing. This contribution provides a foundation for optimizing potential adsorbents and sensing materials for air pollution abatement.

Electrically driven redox process in cerium oxides.

Cerium oxides have attracted much attention because of their uses in three-way catalysts and other catalyst applications. The redox reaction of cerium oxides, as the basis of their use as catalysts, usually takes place at high temperature (>600 K) and/or low oxygen partial pressure. There have been continuous efforts to lower the operating temperatures of cerium oxide further to improve the performance of the catalysts and reduce pollution under the cold-start condition. Here, we report a direct atomic-scale observation of a redox process in cerium oxides driven by an electrical field at ambient temperature. The dynamic changes taking place during the electrically driven redox reaction were imaged by in situ high-resolution transmission electron microscopy, where reversible phase transformations due to the migration of oxygen vacancies have been reproducibly achieved. These results could lead to the low-temperature operation of catalysts for the purification of automobile emissions of pollutants, oxygen generation, and intermediate-temperature solid oxide fuel cells.