Memory properties and charge effect study in Si nanocrystals by scanning capacitance microscopy and spectroscopy

In this letter, isolated Si nanocrystal has been formed by dewetting process with a thin silicon dioxide layer on top. Scanning capacitance microscopy and spectroscopy were used to study the memory properties and charge effect in the Si nanocrystal in ambient temperature. The retention time of trapped charges injected by different direct current (DC) bias were evaluated and compared. By ramp process, strong hysteresis window was observed. The DC spectra curve shift direction and distance was observed differently for quantitative measurements. Holes or electrons can be separately injected into these Si-ncs and the capacitance changes caused by these trapped charges can be easily detected by scanning capacitance microscopy/spectroscopy at the nanometer scale. This study is very useful for nanocrystal charge trap memory application.     

Reduction potentials of heteropolyacid catalysts probed by scanning tunneling microscopy and UV-visible spectroscopy

Reduction potentials of heteropolyacid (HPA) catalysts were probed by scanning tunneling microscopy (STM) and UV-visible spectroscopy. Correlations between reduction potentials and NDR (negative differential resistance) peak voltages, and between reduction potentials and absorption edge energies of HPA catalysts were established. The reduction potentials of HPA catalysts have been shown to follow similar trends to the NDR peak voltages of selfassembled HPA monolayers and the absorption edge energies of bulk HPAs. In the UV-visible spectra of HPA catalysts, lower absorption edge energies corresponded to higher reduction potentials of the HPAs.     

Electronic coupling in fullerene-doped semiconducting carbon nanotubes probed by Raman spectroscopy and electronic transport

We investigate the electronic properties of individual fullerene peapods by combining micro-Raman spectroscopy and (magneto)-transport measurements on the same devices. We bring evidence that the encapsulated C₆₀ molecules strongly modify the electronic band structure of semiconducting nanotubes in the vicinity of the charge neutrality point, including a rigid shift and a partial filling of the energy gap. Using a selective UV excitation of the fullerenes, we demonstrate that the electronic coupling between the contained C₆₀ molecules and the containing carbon nanotube is strongly modified by the partial coalescence of the C₆₀ and their distribution inside the tube. Our experimental results are supported by both numerical simulations of the Density of States and the measured conductance of carbon nanotubes with coalesced fullerenes inside.     

Cone-like graphene nanostructures: electronic and optical properties

A theoretical study of electronic and optical properties of graphene nanodisks and nanocones is presented within the framework of a tight-binding scheme. The electronic densities of states and absorption coefficients are calculated for such structures with different sizes and topologies. A discrete position approximation is used to describe the electronic states taking into account the effect of the overlap integral to first order. For small finite systems, both total and local densities of states depend sensitively on the number of atoms and characteristic geometry of the structures. Results for the local densities of charge reveal a finite charge distribution around some atoms at the apices and borders of the cone structures. For structures with more than 5,000 atoms, the contribution to the total density of states near the Fermi level essentially comes from states localized at the edges. For other energies, the average density of states exhibits similar features to the case of a graphene lattice. Results for the absorption spectra of nanocones show a peculiar dependence on the photon polarization in the infrared range for all investigated structures.     

The synthesis of shape-controlled polypyrrole/graphene and the study of its capacitance properties

Polypyrrole (PPy)/graphene nainocomposite was prepared by methyl orange (MO) reactive template. By changing the amount of MO, the morphology of PPy can be controlled to range from nanoparticle to nanowire. Transmission electron microscopy, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, and X-ray diffraction results demonstrated that the composites were successfully synthesized. The different morphologies of PPy/graphene nanocomposites had certain effects on the supercapacitor performance. The experimental results showed that the capacitance of PPy (nanoparticle)/graphene was higher than that of PPy (nanowire)/graphene. As a model, PPy (nanoparticle)/graphene was used to construct a supercapacitor. By changing the amount of pyrrole monomer, the performance of the supercapacitor prepared from different PPy content was studied in detail.     

Layer-dependent nanoscale electrical properties of graphene studied by conductive scanning probe microscopy

The nanoscale electrical properties of single-layer graphene (SLG), bilayer graphene (BLG) and multilayer graphene (MLG) are studied by scanning capacitance microscopy (SCM) and electrostatic force microscopy (EFM). The quantum capacitance of graphene deduced from SCM results is found to increase with the layer number (n) at the sample bias of 0 V but decreases with n at -3 V. Furthermore, the quantum capacitance increases very rapidly with the gate voltage for SLG, but this increase is much slowed down when n becomes greater. On the other hand, the magnitude of the EFM phase shift with respect to the SiO₂ substrate increases with n at the sample bias of +2 V but decreases with n at -2 V. The difference in both quantum capacitance and EFM phase shift is significant between SLG and BLG but becomes much weaker between MLGs with a different n. The layer-dependent quantum capacitance behaviors of graphene could be attributed to their layer-dependent electronic structure as well as the layer-varied dependence on gate voltage, while the layer-dependent EFM phase shift is caused by not only the layer-dependent surface potential but also the layer-dependent capacitance derivation.     

High-throughput production of pristine graphene in an aqueous dispersion assisted by non-ionic surfactants

The preparation of aqueous graphene dispersions by exfoliation of pristine graphite in the presence of a wide range of surfactants is reported. High graphene concentrations, up to about 1 mg mL⁻¹, were obtained with the use of some non-ionic surfactants. The dispersions consisted of single- and few-layer graphene platelets with their basal planes virtually free of even atomic-sized (point) defects. The potential utility of such highly concentrated dispersions toward the low-cost, large-scale manipulation and processing of graphene was demonstrated by processing them into electrically conductive, free-standing paper-like films.     

Tuning the electronic properties of graphene by hydrogenation in a plasma enhanced chemical vapor deposition reactor

Graphene films grown by chemical vapor deposition on copper foils were hydrogenated using commercially viable methods. Parameters such as plasma power, plasma frequency, and sample temperature were varied to determine the maximum possible hydrogenation without etching the film. The kinetic energy of the ions inside the plasma is critical, in that higher kinetic energy ions tend to etch the film while lower kinetic energy ions participate in the hydrogenation process. The film sheet resistance was shown to increase, while the hole mobility was shown to decrease with increasing hydrogenation. Variable temperature measurements demonstrate a transition from semi-metallic behavior for graphene to semiconducting behavior for hydrogenated graphene. Sheet resistance measurements as a function of temperature also suggest the emergence of a bandgap in the hydrogenated graphene films.     

Investigation of electronic properties of graphene/Si field-effect transistor

We report a high-performance graphene/Si field-effect transistor fabricated via rapid chemical vapor deposition. Oligolayered graphene with a large uniform surface acts as the local gate of the graphene transistors. The scaled transconductance, g_m, of the graphene transistors exceeds 3 mS/μm, and the ratio of the current switch, I_on/I_off, is up to 100. Moreover, the output properties of the graphene transistor show significant current saturation, and the graphene transistor can be modulated using the local graphene gate. These results clearly show that the device is well suited for analog applications.     

Differential nano-bio interactions and toxicity effects of pristine versus functionalized graphene

We report the effect of carboxyl functionalization of graphene in pacifying its strong hydrophobic interaction with cells and associated toxic effects. Pristine graphene was found to accumulate on the cell membrane causing high oxidative stress leading to apoptosis, whereas carboxyl functionalized hydrophilic graphene was internalized by the cells without causing any toxicity.     

Tuning electronic and magnetic properties of partially hydrogenated graphene by biaxial tensile strain: a computational study

Using density functional theory calculations, we have investigated the effects of biaxial tensile strain on the electronic and magnetic properties of partially hydrogenated graphene (PHG) structures. Our study demonstrates that PHG configuration with hexagon vacancies is more energetically favorable than several other types of PHG configurations. In addition, an appropriate biaxial tensile strain can effectively tune the band gap and magnetism of the hydrogenated graphene. The band gap and magnetism of such configurations can be continuously increased when the magnitude of the biaxial tensile strain is increased. This fact that both the band gap and magnetism of partially hydrogenated graphene can be tuned by applying biaxial tensile strain provides a new pathway for the applications of graphene to electronics and photonics.     

Hydrations and water properties in the super salt-resistive gel probed by FT-IR

The so-called “Super Salt-Resistive Gel”, i.e., poly(4-vinyl phenol) (P4VPh) hydrogel, of different water contents (H = 95-40%) was prepared by crosslinking with different amounts of ethylene glycol diglycidyl ether (EGDGE). FT-IR spectroscopy was used to investigate the hydration and hydrogen bond (HB) properties of water in the gel samples. The OH stretching band around 3300 cm⁻¹ was deconvoluted into four sub-bands. On the basis of the relative band area and the peak wave number, it was suggested that HB of water in the gel is most stabilized when the acidic proton of the phenol residue is intact, being free from the chemical crosslinking. Difference spectra for the water band obtained in the presence of salts suggested that only sulfate systems specifically affect polymer hydrations in the gel phase. The sulfate systems were also specific in the perturbation on the main chain CH₂ stretching band; namely, with increasing the salt concentration, the peak showed a significant blue shift, which means that the hydrophobic hydration is stabilized by the typical salting-out divalent anion. All the experimental results on the FT-IR spectroscopy for the P4VPh hydrogel seem to be consistent with our previous ¹H NMR data on the water T₂ (Sakai Y, Kuroki S, Satoh M. Langmuir 2008; 24:6981-7.) as well as the specific stabilization of water (HB and hydration) in the gel that has been suggested on the basis of the swelling behavior.     

Microviscosity of an ion-conducting polymer probed by fluorescence depolarization and dielectric spectroscopy

The local segmental motions of polypropylene oxide melt with variable amounts of added LiClO₄ salt (0.3-2.2 mol% per repeat unit) were studied using dielectric relaxation spectroscopy and single photon counting methods after two-photon excitation of an unattached fluorescent dye, rhodamine123, embedded at low concentration within the polymer. The lithium ions serve as junctions between polymer chains and increase the bulk viscosity. The measured microviscosity contains information on the local environment of ClO₄⁻ ions.     

Dynamics of pristine graphite and graphene at an air‐water interface

We examine the dynamics and morphology of graphitic films at an air‐water interface in a Langmuir trough by varying interfacial surface coverage, by observing in situ interfacial structure, and by characterizing interfacial structure of depositions on mica substrates. In situ interfacial structure is visualized with Brewster angle microscopy and depositions of the interface are characterized with atomic force microscopy and field‐emission scanning electron microscopy. Compression/expansion curves exhibit a monotonically decreasing surface pressure between consecutive compressions, but demonstrate a “rebound” of hysteretic behavior when the interface is allowed to relax between consecutive compressions. This dynamic results from a competition between consolidation of the interface via agglomeration of particles or the stacking of graphene sheets, and a thermally‐driven relaxation where nanometer‐thick particles are able to overcome capillary interactions. These results are especially relevant to applications where functional films with controlled conductivity and transparency may be produced via liquid‐phase deposition methods. © 2018 American Institute of Chemical Engineers AIChE J, 64: 3177–3187, 2018     

The effects of interlayer mismatch on electronic properties of bilayer armchair graphene nanoribbons

We investigate the impact of interlayer mismatch on the electronic properties of bilayer graphene nanoribbons (BGNRs) with armchair-edges in terms of the total energy and electronic structures by first principle calculations. Simulation results show that in-plane misalignments require little energy and a large variation in the energy bandgap (E_G) can be observed. Based on the resulting atomic configurations due to the misalignments, the details of the observed relationship between bandgap and the lattice mismatch are investigated. It is observed that in general, misalignment in the transverse direction results in a decrease in the interaction between the two layers, giving rise to a larger E_G. On the other hand, misalignment in the longitudinal direction, i.e. along the edges, leads to an oscillation in E_G due to the periodic change of the GNR stacking order. A combination of these movements results in a complex variation of E_G, which introduces great uncertainty in electronic devices. However, such a phenomenon could also be used in various kinds of nanoelectromechanical systems as it provides a large change in electronic properties with a small movement.     

Real-time investigation of crystallization in nylon 6-clay nano-composite probed by infrared spectroscopy

Via time-resolved FTIR, we examined the real-time investigation of the structural change in molecular chain of nylon 6 during crystallization of neat nylon 6 and the corresponding nano-composite (N6C3.7) having fully exfoliated structure. The neat nylon 6 predominantly formed α-phase in the crystallization temperature (T_c) range of 155-195 °C. For N6C3.7 crystallization at low T_c range of 150-168 °C, where the network structure formed by the dispersed clay particles still affected chain folding of nylon 6, the formation of the γ-phase was dominant. The crystallization took place so rapidly (less than 1 s) without induction time of crystallization. At high T_c range (=177-191 °C), the stable growth of the α-phase crystal coexisting with γ-phase occurred in N6C3.7 crystallization. The growth mechanism in the subsequent crystallization processes (amides IIIα and IIIγ) was virtually the same in both N6C3.7 and neat nylon 6.     

The unfolding and folding dynamics of TNfnALL probed by single molecule force-ramp spectroscopy

Tenascin, an important extracellular matrix protein, is subject to stretching force under physiological conditions and plays important roles in regulating the cell-matrix interactions. Using the recently developed single molecule force-ramp spectroscopy, we investigated the unfolding-folding kinetics of a recombinant tenascin fragment TNfnALL. Our results showed that all the 15 FnIII domains in TNfnALL have similar spontaneous unfolding rate constant at zero force, but show great difference in their folding rate constants. Our results demonstrated that single molecule force-ramp spectroscopy is a powerful tool for accurate determination of the kinetic parameters that characterize the unfolding and folding reactions. We anticipate that single molecule force-ramp spectroscopy will become a versatile addition to the single molecule manipulation tool box and greatly expand the scope of single molecule force spectroscopy.     

Study of structure and thermal properties of polypropylene and chlorinated polypropylene by infrared spectroscopy and differential scanning calorimetry

The infrared spectra of chlorinated polypropylene (CPP) are interpreted and tentative assignments of C—Cl stretching vibration of CPP are proposed. Recently, infrared spectrophotometry has been applied as a useful tool for the investigation of polymer transitions. The solid-state transition in isotactic polypropylene (IPP) was studied by infrared method in the range of ?50° to 30°C; and from the temperature dependence of the peak absorbance, transition at -10°C was detected. This temperature of -10°C agrees well with the T_g2 detected by other methods. From these results, it is presumed that T_g2 is attributed to a motion (thermal expansion) of IPP segments in either crystalline and amorphous region. The thermal transition of chlorinated isotactic polypropylene (CIPP) was also examined with differential scanning calorimetry and infrared method, and two thermal transitions were observed. A higher transition (T_H) has a minimum at a degree of chlorination of about 39 wt-%; and a lower transition (T_L) changes linearly with increasing degree of chlorination. Infrared results indicate that T_H may be associated with a motion (thermal expansion) in chlorinated segments, and T_L may be associated with a motion in unchlorinated segments. These results of infrared studies also suggest that CIPP may have a block structure.