Anderson Localization of Phonons in Thermally Superinsulating Graphene Aerogels with Metal-Like Electrical Conductivity.

In the quest to improve energy efficiency and design better thermal insulators, various engineering strategies have been extensively investigated to minimize heat transfer through a material. Yet, the suppression of thermal transport in a material remains elusive because heat can be transferred by multiple energy carriers. Here, the realization of Anderson localization of phonons in a random 3D elastic network of graphene is reported. It is shown that thermal conductivity in a cellular graphene aerogel can be drastically reduced to 0.9 mW m-1 K-1 by the application of compressive strain while keeping a high metal-like electrical conductivity of 120 S m-1 and ampacity of 0.9 A. The experiments reveal that the strain can cause phonon localization over a broad compression range. The remaining heat flow in the material is dominated by charge transport. Conversely, electrical conductivity exhibits a gradual increase with increasing compressive strain, opposite to the thermal conductivity. These results imply that strain engineering provides the ability to independently tune charge and heat transport, establishing a new paradigm for controlling phonon and charge conduction in solids. This approach will enable the development of a new type of high-performance insulation solutions and thermally superinsulating materials with metal-like electrical conductivity.

Terahertz charge transport dynamics in 3D graphene networks with localization and band regimes.

Terahertz steady-state and time-resolved conductivity and permittivity spectra were measured in 3D graphene networks assembled in free-standing covalently cross-linked graphene aerogels. Investigation of a transition between reduced-graphene oxide and graphene controlled by means of high-temperature annealing allowed us to elucidate the role of defects in the charge carrier transport in the materials. The THz spectra reveal increasing conductivity and decreasing permittivity with frequency. This contrasts with the Drude- or Lorentz-like conductivity typically observed in various 2D graphene samples, suggesting a significant contribution of a relaxational mechanism to the conductivity in 3D graphene percolated networks. The charge transport in the graphene aerogels exhibits an interplay between the carrier hopping among localized states and a Drude contribution of conduction-band carriers. Upon photoexcitation, carriers are injected into the conduction band and their dynamics reveals picosecond lifetime and femtosecond dephasing time. Our findings provide important insight into the charge transport in complex graphene structures.

Universal Strategy for Reversing Aging and Defects in Graphene Oxide for Highly Conductive Graphene Aerogels.

The production of highly stable, defect-free, and electrically conducting 3D graphene structures from graphene oxide precursors is challenging. This is because graphene oxide is a metastable material whose structure and chemistry evolve due to aging. Aging changes the relative composition of oxygen functional groups attached to the graphene oxide and negatively impacts the fabrication and properties of reduced graphene oxide. Here, we report a universal strategy to reverse the aging of graphene oxide precursors using oxygen plasma treatment. This treatment decreases the size of graphene oxide flakes and restores negative zeta potential and suspension stability in water, enabling the fabrication of compact and mechanically stable graphene aerogels using hydrothermal synthesis. Moreover, we employ high-temperature annealing to remove oxygen-containing functionalities and repair the lattice defects in reduced graphene oxide. This method allows obtaining highly electrically conducting graphene aerogels with electrical conductivity of 390 S/m and low defect density. The role of carboxyl, hydroxyl, epoxide, and ketonic oxygen species is thoroughly investigated using X-ray photoelectron and Raman spectroscopies. Our study provides unique insight into the chemical transformations occurring during the aging and thermal reduction of graphene oxide from room temperature up to 2700 °C.

Electrostatic Gating of Monolayer Graphene by Concentrated Aqueous Electrolytes.

Electrostatic gating using electrolytes is a powerful approach for controlling the electronic properties of atomically thin two-dimensional materials such as graphene. However, the role of the ionic type, size, and concentration and the resulting gating efficiency is unclear due to the complex interplay of electrochemical processes and charge doping. Understanding these relationships facilitates the successful design of electrolyte gates and supercapacitors. To that end, we employ in situ Raman microspectroscopy combined with electrostatic gating using various concentrated aqueous electrolytes. We show that while the ionic type and concentration alter the initial doping state of graphene, they have no measurable influence over the rate of the doping of graphene with applied voltage in the high ionic strength limit of 3-15 M. Crucially, unlike for conventional dielectric gates, a large proportion of the applied voltage contributes to the Fermi level shift of graphene in concentrated electrolytes. We provide a practical overview of the doping efficiency for different gating systems.

High-Temperature Fire Resistance and Self-Extinguishing Behavior of Cellular Graphene.

The ability to protect materials from fire is vital to many industrial applications and life safety systems. Although various chemical treatments and protective coatings have proven effective as flame retardants, they provide only temporary prevention, as they do not change the inherent flammability of a given material. In this study, we demonstrate that a simple change of the microstructure can significantly boost the fire resistance of an atomically thin material well above its oxidation stability temperature. We show that free-standing graphene layers arranged in a three-dimensional (3D) cellular network exhibit completely different flammability and combustion rates from a graphene layer placed on a substrate. Covalently cross-linked cellular graphene aerogels can resist flames in air up to 1500 °C for a minute without degrading their structure or properties. In contrast, graphene on a substrate ignites immediately above 550 °C and burns down in a few seconds. Raman spectroscopy, X-ray photoelectron spectroscopy, and thermogravimetric studies reveal that the exceptional fire-retardant and self-extinguishing properties of cellular graphene originate from the ability to prevent carbonyl defect formation and capture nonflammable carbon dioxide gas in the pores. Our findings provide important information for understanding graphene's fire-retardant mechanism in 3D structures/assemblies, which can be used to enhance flame resistance of carbon-based materials, prevent fires, and limit fire damage.

The Effects of Ultrasound Treatment of Graphite on the Reversibility of the (De)Intercalation of an Anion from Aqueous Electrolyte Solution.

Low cycling stability is one of the most crucial issues in rechargeable batteries. Herein, we study the effects of a simple ultrasound treatment of graphite for the reversible (de)intercalation of a ClO4- anion from a 2.4 M Al(ClO4)3 aqueous solution. We demonstrate that the ultrasound-treated graphite offers the improved reversibility of the ClO4- anion (de)intercalation compared with the untreated samples. The ex situ and in situ Raman spectroelectrochemistry and X-ray diffraction analysis of the ultrasound-treated materials shows no change in the interlayer spacing, a mild increase in the stacking order, and a large increase in the amount of defects in the lattice accompanied by a decrease in the lateral crystallite size. The smaller flakes of the ultrasonicated natural graphite facilitate the improved reversibility of the ClO4- anion electrochemical (de)intercalation and a more stable electrochemical performance with a cycle life of over 300 cycles.

Contact resistance based tactile sensor using covalently cross-linked graphene aerogels.

A movable electrical contact between two materials is one of the most fundamental, simple, and common components in electronics that is used for binary control of a conducting path in an electrical circuit. Here, variable contact resistance between a highly elastic graphene aerogel and a rigid metal electrode is used for the analysis of non-binary pushing and pulling mechanical forces acting on the contact, enabling superior strain and pressure measurements. The variable contact resistance based electromechanical sensors demonstrate superfast, ultrasensitive and quantitative measurements of compressive and tensile stress from -1.18 MPa to 0.55 MPa. The sensors can operate over the temperature range of -60 to 100 °C, cover the whole skin and human motion range, and determine the weight of a grasped object. The measurement of such high forces has only been possible due to the high-temperature induced covalent cross-linking of graphene in the aerogel that provides high strength, durability, and fast response (<0.5 ms) to the sensing element. The study demonstrates the great potential of the contact resistance-controlled sensing, which enables high-precision and reliable measurement of strain and pressure over a remarkable large sensing range, providing new opportunities for applications in human-machine interfaces, robotics, flexible electronics, and haptic technology.

Nucleation and growth of metal-catalyzed silicon nanowires under plasma.

We report the results of a microscopic study of the nucleation and early growth stages of metal-catalyzed silicon nanowires in plasma-enhanced chemical vapor deposition. The nucleation of silicon nanowires is investigated as a function of different deposition conditions and metal catalysts (Sn, In and Au) using correlation of atomic force microscopy and scanning electron microscopy. This correlation method enabled us to visualize individual catalytic nanoparticles before and after the nanowire growth and identify the key parameters influencing the nanowire nucleation under plasma. The size and position of catalytic nanoparticles are found to play a significant role in the nucleation. We demonstrate that only small isolated nanoparticles in the range of 10-20 nm contribute to the nanowire growth under plasma, while larger nanoparticles are inactive because they get buried under a layer of a-Si:H before reaching supersaturation. Systematic analysis of different growth parameters reveals that the nanowire growth in plasma contradicts the vapor-liquid-solid mechanism at thermal equilibrium in many ways. The nanowire growth is much faster and proceeds even at negligible silicon solubility and bellow the eutectic temperature of the metal-silicon alloy. Based on the observations, we propose the nanowire growth under plasma to be characterized by the rapid solidification mechanism, where a crystalline silicon phase emerges from a metastable supersaturated liquid metal-silicon phase in local nonequilibrium.

Molecular detection by liquid gated Hall effect measurements of graphene.

Conventional electrical biosensing techniques include Cyclic Voltammetry (CV, amperometric) and ion-sensitive field effect transistors (ISFETs, potentiometric). However, CV is not able to detect electrochemically inactive molecules where there is no redox reaction in solution, and the resistance change in pristine ISFETs in response to low concentration solutions is not observable. Here, we show a very sensitive label-free biosensing method using Hall effect measurements on unfunctionalized graphene devices where the gate electrode is immersed in the solution containing the analyte of interest. This liquid gated Hall effect measurement (LGHM) technique is independent of redox reactions, and it enables the extraction of additional information regarding electrical properties from graphene as compared with ISFETs, which can be used to improve the sensitivity. We demonstrate that LGHM has a higher sensitivity than conventional biosensing methods for l-histidine in the pM range. The detection mechanism is proposed to be based on the interaction between the ions and graphene. The ions could induce asymmetry in electron-hole mobility and inhomogeneity in graphene, and they may also respond to the Hall effect measurement. Moreover, the calculation of capacitance values shows that the electrical double layer capacitance is dominant at relatively high gate voltages in our system, and this is useful for applications including biosensing, energy storage, and neural stimulation.

Molecular Dipole-Driven Electronic Structure Modifications of DNA/RNA Nucleobases on Graphene.

The emergence of graphene in recent years provides exciting avenues for achieving fast, reliable DNA/RNA sensing and sequencing. Here we explore the possibility of enhancing electronic fingerprints of nucleobases adsorbed on graphene by tuning the surface coverage and modifying molecular dipoles using first-principles calculations. We demonstrate that intermolecular interactions have a strong influence on the adsorption geometry and the electronic structure of the nucleobases, resulting in tilted configurations and a considerable modification of their electronic fingerprints in graphene. Our analysis reveals that the molecular dipole of the nucleobase molecules plays a dominant role in the electronic structure of graphene-nucleobase systems, inducing significant changes in the work functions and energy level alignments at the interface. These results highlight tunable control of the measured molecular signals in graphene by optimizing the surface contact between nucleobases and graphene. Our findings have important implications for identification and understanding of molecular fingerprints of DNA/RNA nucleobases in graphene-based sensing and sequencing methods.

Direct fabrication of 3D graphene on nanoporous anodic alumina by plasma-enhanced chemical vapor deposition.

High surface area electrode materials are of interest for a wide range of potential applications such as super-capacitors and electrochemical cells. This paper describes a fabrication method of three-dimensional (3D) graphene conformally coated on nanoporous insulating substrate with uniform nanopore size. 3D graphene films were formed by controlled graphitization of diamond-like amorphous carbon precursor films, deposited by plasma-enhanced chemical vapour deposition (PECVD). Plasma-assisted graphitization was found to produce better quality graphene than a simple thermal graphitization process. The resulting 3D graphene/amorphous carbon/alumina structure has a very high surface area, good electrical conductivity and exhibits excellent chemically stability, providing a good material platform for electrochemical applications. Consequently very large electrochemical capacitance values, as high as 2.1 mF for a sample of 10 mm(3), were achieved. The electrochemical capacitance of the material exhibits a dependence on bias voltage, a phenomenon observed by other groups when studying graphene quantum capacitance. The plasma-assisted graphitization, which dominates the graphitization process, is analyzed and discussed in detail.

A graphene field-effect transistor as a molecule-specific probe of DNA nucleobases.

Fast and reliable DNA sequencing is a long-standing target in biomedical research. Recent advances in graphene-based electrical sensors have demonstrated their unprecedented sensitivity to adsorbed molecules, which holds great promise for label-free DNA sequencing technology. To date, the proposed sequencing approaches rely on the ability of graphene electric devices to probe molecular-specific interactions with a graphene surface. Here we experimentally demonstrate the use of graphene field-effect transistors (GFETs) as probes of the presence of a layer of individual DNA nucleobases adsorbed on the graphene surface. We show that GFETs are able to measure distinct coverage-dependent conductance signatures upon adsorption of the four different DNA nucleobases; a result that can be attributed to the formation of an interface dipole field. Comparison between experimental GFET results and synchrotron-based material analysis allowed prediction of the ultimate device sensitivity, and assessment of the feasibility of single nucleobase sensing with graphene.

Spin-orbit interaction in a two-dimensional hole gas at the surface of hydrogenated diamond.

Hydrogenated diamond possesses a unique surface conductivity as a result of transfer doping by surface acceptors. Yet, despite being extensively studied for the past two decades, little is known about the system at low temperature, particularly whether a two-dimensional hole gas forms at the diamond surface. Here we report that (100) diamond, when functionalized with hydrogen, supports a p-type spin-3/2 two-dimensional surface conductivity with a spin-orbit interaction of 9.74 ± 0.1 meV through the observation of weak antilocalization effects in magneto-conductivity measurements at low temperature. Fits to 2D localization theory yield a spin relaxation length of 30 ± 1 nm and a spin-relaxation time of ∼ 0.67 ± 0.02 ps. The existence of a 2D system with spin orbit coupling at the surface of a wide band gap insulating material has great potential for future applications in ferromagnet-semiconductor and superconductor-semiconductor devices.

Graphene field effect transistor as a probe of electronic structure and charge transfer at organic molecule-graphene interfaces.

The electronic structure of physisorbed molecules containing aromatic nitrogen heterocycles (triazine and melamine) on graphene is studied using a combination of electronic transport, X-ray photoemission spectroscopy and density functional theory calculations. The interfacial electronic structure and charge transfer of weakly coupled molecules on graphene is found to be governed by work function differences, molecular dipole moments and polarization effects. We demonstrate that molecular depolarization plays a significant role in these charge transfer mechanisms even at submonolayer coverage, particularly for molecules which possess strong dipoles. Electronic transport measurements show a reduction of graphene conductivity and charge carrier mobility upon the adsorption of the physisorbed molecules. This effect is attributed to the formation of additional electron scattering sites in graphene by the molecules and local molecular electric fields. Our results show that adsorbed molecules containing polar functional groups on graphene exhibit different coverage behaviour to nonpolar molecules. These effects open up a range of new opportunities for recognition of different molecules on graphene-based sensor devices.

Coupling of a single-photon emitter in nanodiamond to surface plasmons of a nanochannel-enclosed silver nanowire.

A finite element method is applied to study the coupling between a nitrogen vacancy (NV) single photon emitter in nanodiamond and surface plasmons in a silver nanowire embedded in an alumina nanochannel template. We investigate the effective parameters in the coupled system and present detailed optimization for the maximum transmitted power at a selected optical frequency (650 nm). The studied parameters include nanowire length, nanowire diameter, distance between the dipole and the nanowire, orientation of the emitter and refractive index of the surrounding. It is found that the diameter of the nanowire has a strong influence on the propagation of the surface plasmon polaritons and emission power from the bottom and top endings of the nanowire.

Development of a templated approach to fabricate diamond patterns on various substrates.

We demonstrate a robust templated approach to pattern thin films of chemical vapor deposited nanocrystalline diamond grown from monodispersed nanodiamond (mdND) seeds. The method works on a range of substrates, and we herein demonstrate the method using silicon, aluminum nitride (AlN), and sapphire substrates. Patterns are defined using photo- and e-beam lithography, which are seeded with mdND colloids and subsequently introduced into microwave assisted chemical vapor deposition reactor to grow patterned nanocrystalline diamond films. In this study, we investigate various factors that affect the selective seeding of different substrates to create high quality diamond thin films, including mdND surface termination, zeta potential, surface treatment, and plasma cleaning. Although the electrostatic interaction between mdND colloids and substrates is the main process driving adherence, we found that chemical reaction (esterification) or hydrogen bonding can potentially dominate the seeding process. Leveraging the knowledge on these different interactions, we optimize fabrication protocols to eliminate unwanted diamond nucleation outside the patterned areas. Furthermore, we have achieved the deposition of patterned diamond films and arrays over a range of feature sizes. This study contributes to a comprehensive understanding of the mdND-substrate interaction that will enable the fabrication of integrated nanocrystalline diamond thin films for microelectronics, sensors, and tissue culturing applications.

Strong oxidation resistance of atomically thin boron nitride nanosheets.

Investigation of oxidation resistance of two-dimensional (2D) materials is critical for many of their applications because 2D materials could have higher oxidation kinetics than their bulk counterparts due to predominant surface atoms and structural distortions. In this study, the oxidation behavior of high-quality boron nitride (BN) nanosheets of 1-4 layers thick has been examined by heating in air. Atomic force microscopy and Raman spectroscopy analyses reveal that monolayer BN nanosheets can sustain up to 850 °C, and the starting temperature of oxygen doping/oxidation of BN nanosheets only slightly increases with the increase of nanosheet layer and depends on heating conditions. Elongated etch lines are found on the oxidized monolayer BN nanosheets, suggesting that the BN nanosheets are first cut along the chemisorbed oxygen chains and then the oxidative etching grows perpendicularly to these cut lines. The stronger oxidation resistance of BN nanosheets makes them more preferable for high-temperature applications than graphene.