Spectroscopic studies of large sheets of graphene oxide and reduced graphene oxide monolayers prepared by Langmuir-Blodgett technique

Graphene oxide (GO) sheets prepared by chemical exfoliation were spread at the air-water interface and transferred to silicon substrates by Langmuir-Blodgett technique as closely spaced monolayers of 20-40 μm size. Hydrazine exposure followed by annealing in vacuum and argon ambient results in the formation of reduced graphene oxide (RGO) monolayers, without significantly affecting the overall morphology of the sheets. The monolayer character of both GO and RGO sheets was ascertained by atomic force microscopy. X-ray photoelectron spectroscopy supported by Fourier transform infrared spectroscopy revealed that the reduction process results in a significant decrease in oxygen functionalities, accompanied by a substantial decrease in the ratio of non-graphitic to graphitic (sp² bonded) carbon in the monolayers from 1.2 to 0.35. Raman spectra of GO and RGO monolayers have shown that during the reduction process, the G-band shifts by 8-12 cm^− 1 and the ratio of the intensities of D-band to G-band, I(D)/I(G) decreases from 1.3 ± 0.3 to 0.8 ± 0.2, which is in tune with the smaller non-graphitic carbon content of RGO monolayers. The significant decrease in I(D)/I(G) has been explained by assuming that substantial order is present in precursor GO monolayers as well as RGO monolayers obtained by solid state reduction.     

Preparation and characterization of CdS nanocrystallites in nafion

Perfluoroethylene sulfonic acid polymer (NAFION) films are subjected to ion exchange in the medium of aqueous solutions of cadmium acetate, followed by ammonia passivation. The films are then treated with hydrogen sulfide gas for prescribed times. X-ray powder diffraction data of these samples have been analyzed for estimating the sizes of the nanocrystallites. The optical absorption spectra of the samples show an absorption edge beginning at 525 nm for the largest size clusters. A broad absorption band appears with a maximum around 410 nm–440 nm in the smaller size clusters.     

Stark spectroscopy of Langmuir-Blodgett monolayers and multilayers

The electrical (capacity and conductivity) and electro-optical (Stark spectra) properties were investigated for monolayers of an anthraquinone dye. The polar structure of the monolayers results in the appearance of the linear Stark effect with a field-induced change δT in the optical transmission proportional to the difference δμ of the dipole moments for an excited and the ground molecular states. Multilayers were shown to be non-polar owing to the effects of molecular association. In the latter case only the quadratic Stark effect is observed with δT proportional to the difference δ of the polarizabilities of an associated excited state and the ground state. It was also shown that the electric field distribution across an ultrathin Langmuir-Blodgett film is extremely non-uniform.     

Aggregation of cyanine dyes at Langmuir-Blodgett monolayers

Cyanine dyes are adsorbed from aqueous solution to Langmuir-Blodgett (LB) films, forming monolayers of Scheibe or J aggregates, whereas the dye in solution is in monomeric state. As well as J aggregates other types of aggregates (H, H^* aggregates, oligomers, dimers) can form and interchange at the LB interface depending on conditions and the structures of the dyes and amphiphiles. Reflection spectroscopy proved to be a useful method for studying aggregates and kinetic processes at the air-water interface as the bulk subphase is not recorded by this method.

Aggregation of dyes seems to be controlled by Coulomb interactions and the surface structure of the interacting monolayers. Cationic cyanine dyes are adsorbed only at negatively charged monolayers (e.g. fatty acids) whereas cyanine dyes with negative net charge form aggregates only at positively charged monolayers (e.g. amphiphilic ammonium salts).

Aggregation of cyanine dyes has also been found at LB monolayers of amphiphilic nucleic acids which have been synthesized for the first time. Aggregates of adsorbed cyanine dyes can be transferred to solid substrates by the LB method and investigated by other spectroscopic methods or microscopy.

Adsorption of cyanine dyes at vesicle bilayers has been reported; this was studied by absorption spectroscopy of vesicular dye solutions. Comparison of these spectra with reflection spectra of corresponding LB films reveals significant differences between the effects in LB monolayers and those in vesicular bilayers.     

Converting graphene oxide monolayers into boron carbonitride nanosheets by substitutional doping

To realize graphene-based electronics, bandgap opening of graphene has become one of the most important issues that urgently need to be addressed. Recent theoretical and experimental studies show that intentional doping of graphene with boron and nitrogen atoms is a promising route to open the bandgap, and the doped graphene might exhibit properties complementary to those of graphene and hexagonal boron nitride (h-BN), largely extending the applications of these materials in the areas of electronics and optics. This work demonstrates the conversion of graphene oxide nanosheets into boron carbonitride (BCN) nanosheets by reacting them with B(2) O(3) and ammonia at 900 to 1100 °C, by which the boron and nitrogen atoms are incorporated into the graphene lattice in randomly distributed BN nanodomains. The content of BN in BN-doped graphene nanosheets can be tuned by changing the reaction temperature, which in turn affects the optical bandgap of these nanosheets. Electrical measurements show that the BN-doped graphene nanosheet exhibits an ambipolar semiconductor behavior and the electrical bandgap is estimated to be ≈25.8 meV. This study provides a novel and simple route to synthesize BN-doped graphene nanosheets that may be useful for various optoelectronic applications.     

Highly conducting graphene sheets and Langmuir-Blodgett films

Graphene is an intriguing material with properties that are distinct from those of other graphitic systems. The first samples of pristine graphene were obtained by 'peeling off' and epitaxial growth. Recently, the chemical reduction of graphite oxide was used to produce covalently functionalized single-layer graphene oxide. However, chemical approaches for the large-scale production of highly conducting graphene sheets remain elusive. Here, we report that the exfoliation-reintercalation-expansion of graphite can produce high-quality single-layer graphene sheets stably suspended in organic solvents. The graphene sheets exhibit high electrical conductance at room and cryogenic temperatures. Large amounts of graphene sheets in organic solvents are made into large transparent conducting films by Langmuir-Blodgett assembly in a layer-by-layer manner. The chemically derived, high-quality graphene sheets could lead to future scalable graphene devices.     

Plasmon-enhanced resonance Raman scattering and fluorescence in Langmuir-Blodgett monolayers

Localized surface plasmon resonances in silver and gold nanostructures are engaged to enhance the inelastic Raman scattering and the fluorescence of a phopholipid containing a sulforhodamine 101 acid chloride dye known as Texas Red. The most efficient coupling and enhancement are attained when the excitation laser line is in resonance with both the chromophore and the plasmon absorption of the nanostructure, the case of surface-enhanced resonance Raman scattering, allowing single-molecule detection. The tagged phospholipid was incorporated into a single fatty acid Langmuir monolayer at varying concentrations and transferred onto an evaporated Ag nanoparticle film. Surface-enhanced fluorescence is achieved using shell-isolated silica-coated gold nanoparticles, an emission enhancement named SHINEF.     

Single-molecule detection using surface-enhanced resonance Raman scattering and Langmuir-Blodgett monolayers

The Langmuir-Blodgett (LB) technique has been used to obtain spatially resolved surface-enhanced resonance Raman scattering (SERRS) spectra of single dye molecules dispersed in the matrix of a fatty acid. The experimental results presented here mimic the original electrochemical surface-enhanced Raman scattering (SERS) work where the background bulk water did not interfere with the detection of the SERS signal of molecules adsorbed onto the rough silver electrode. LB monolayers of the dye in fatty acid have been fabricated on silver island films with a concentration, in average, of one probe molecule per micrometer square. The properties of single-molecule spectroscopy were investigated using micro-Raman including mapping and global images. Blinking of the SERRS signal was also observed.     

Domain and network aggregation of CdTe quantum rods within Langmuir-Blodgett monolayers

Control over the organization of quantum rods was demonstrated by changing the surface area at the air-liquid interface by means of the Langmuir-Blodgett (LB) technique. The LB isotherm of CdTe quantum rods capped with a mixture of alkylphosphines shows a transition point in the liquid-solid state, which is caused by the inter-rod reorganization. As we observed, at low surface pressure the quantum rods are assembled into round-shaped aggregates composed of a monolayer of nanorods packed in limited-size clusters with random orientation. The increase of the surface pressure leads to the rearrangement of these aggregates into elongated bundles composed of uniformly oriented nanorod clusters. Further compression results in denser packing of nanorods aggregates and in the transformation of monolayered domains into a continuous network of locally ordered quantum rods.     

Self-assembled 1-octadecanethiol monolayers on graphene for mercury detection

We report studies on surface modification of graphene with 1-octadecanethiol and its application as heavy metal sensors. The alkanethiol molecules can self-assemble into large-scale highly ordered monolayers on single-layer graphene regardless of the roughness of graphene surfaces inherited from the underlying amorphorous silicon oxide (SiO2) dielectric substrates. Atomically resolved scanning tunneling microscopy imaging of modified graphene sheets on SiO2 was conducted to reveal configuration details of the self-assembled structure. Functionalization of graphene field effect transistors (Gra-FETs) with 1-octadecanethiol was realized and successfully explored for mercury(II) (Hg2+) detection at 10 ppm.     

Electronic transport in two stacked graphene monolayers

We report on interlayer and lateral electronic transport measurements in two stacked graphene monolayers which have separate electrical contacts. The current-voltage characteristic across the two layers shows linear Ohmic behavior at zero magnetic field. At high magnetic fields, sequences of quantum Hall plateaus of the overlap region with filling factors 4, 8, and 12 are observed which can be explained by equilibration of the edge channel potentials of the individual graphene layers. An anomaly is observed at total filling factors ±2 in the overlap region. The I-V characteristic for interlayer transport turns nonlinear, and the Hall signal vanishes, indicating a magnetic field induced electrical decoupling of the two graphene layers.     

Antimony-doped tin oxide nanocrystallites prepared by a combustion process

Nanocrystalline antimony-doped tin oxide (ATO) has been successfully synthesized for the first time by the combustion process, using nitrate as an oxidizer and citrate as a fuel, and the starting metal sources are granulated tin and Sb₂O₃. The ATO crystallites are characterized by means of thermogravimetry differential scanning calorimetry (TG-DSC), X-ray diffraction (XRD), Brunauer, Emmett, and Teller adsorption (BET) and transmission electron microscopy (TEM). It is found that the fuel-to-oxidizer molar ratio has great effect on the quality of the ATO nanocrystallites. The average crystallite size decreases with the increase of fuel-to-oxidizer molar ratio, while the specific surface area shows a linearly increasing tendency. Crystallites with an average size of 15.2 nm and the highest surface area 39.7 m² g⁻¹ are obtained when the fuel-to-oxidizer ratio is about 0.41.     

Reduced graphene oxide

This paper describes a novel approach to the preparation of reduced graphene oxide and its dispersions in organic solvents. Graphite oxide, graphene oxide, and reduced graphene oxide have been prepared and characterized by a variety of physicochemical techniques.     

Effect of heat treatment on the photoluminescence of CdS nanocrystallites in cadmium-rich organic Langmuir Blodgett matrix

CdS nanocrystallites were grown within an organic layered matrix by partial sulphidation of precursor cadmium arachidate LB multilayers. The cadmium arachidate–arachidic acid composite multilayers containing CdS nanocrystallites were enriched by intercalation with Cd²⁺ ions in aqueous solution of CdCl₂ and subsequently heat treated at different temperatures up to 300 °C, in air and in vacuum. CdS nanocrystallites within the composite multilayer have been found to exhibit treatment process dependent characteristic changes in optical absorption and luminescence. The optical data obtained at different stages/conditions of processing have been analyzed by considering changes in excitonic absorption and emission as well as contributions from surface and bulk defects related emission and a suitable energy level diagram has been proposed. The composition, microstructure and surface morphology of the composite multilayers were also studied at all stages of processing to develop a comprehensive understanding of the interaction of the organic matrix with CdS nanocrystallites and the consequent influence on the optical behavior of the nanocrystallites. These studies have shown that the organic moieties encapsulating the CdS nanocrystallites tend to restrict their growth and aggregation, while the presence of cadmium in the organic matrix is responsible for the passivation of surface defects as well as the reduction of bulk defects, and these factors have significant influence on the photoluminescence of CdS nanocrystallites.     

Growth of dielectric-embedded silicon nanocrystallites for light-emitting device application

Dielectric films with embedded silicon Si nanocrystallites (Si-Nc) have been recognized as promising light-emitting materials for future integrated photonics based on silicon technology. This work reports a novel method of making this kind of material by high-temperature annealing of Si-rich oxide or nitride films which gives rise to the phase separation reaction and the formation of crystalline silicon nanoclusters in the films. Various characteristics of these materials were studied in detail by using transmission electron microscope, X-ray photoelectron spectroscopy (XPS), Raman, and photoluminescence (PL). Strong transverse optical (TO) mode of Si-Nc at around 516 cm(-1) was found in the Raman spectra of the annealed dielectric films. XPS studies indicate that the Si 2p spectra could be transformed from a random bonding structure (as-deposited) to a random mixing of Si-Nc with stoichiometric oxide or nitride phase after the high-temperature annealing. The energy locations of PL were found to depend on the amount of rich Si and the annealing conditions. Longer and higher temperature annealing can result in the growth of the Si-Nc size and leads to a red-shift of PL. Direct correlation among the crystallite sizes with the PL peaks was found.     

Synthesis of vanadium-doped tin oxide nanocrystallites for CO gas sensing

Vanadium-doped tin oxide nanoparticles have been synthesized by a hydrolysis and co-precipitation method from vanadium(III) acetylacetonate and tin tetrachloride. Addition of vanadium species into the tin oxide matrix resulted in a decrease in the grain size, as indicated by X-ray diffraction (XRD) and electron microscopy. X-ray photoelectron spectroscopy (XPS) revealed an interaction between tin and vanadium atoms in the mixed oxide structure and the presence of oxygen vacancies and vanadium cations with multiple oxidation states in the surface region. The binary solid solution exhibited a higher sensitivity to CO gas than pure tin oxide, after both were evaluated in a semiconductor gas sensor. Results demonstrate the feasibility of vanadium dopant for use to enhance sensor response.     

Synthesis of iron-doped vanadium-tin oxide nanocrystallites for CO gas sensing

Iron-doped vanadium-tin oxide nanoparticles have been synthesized by a hydrolysis and co-precipitation method from iron(II) acetate, vanadium(III) acetylacetonate and tin tetrachloride. The mixed oxide was characterized as a tetragonal cassiterite structure by X-ray diffraction (XRD). X-ray photoelectron spectroscopy (XPS) revealed an electronic interaction between tin, vanadium and iron atoms in the oxide structure. Addition of iron species into the vanadium-tin oxide led to a decrease in the crystallite size and changes in the oxidation states of vanadium and iron cations in the surface region. Based on sensitivity measurements in a semiconductor CO gas sensor, the iron doping resulted in a shift of the maximum sensitivity toward the lower temperature side. A correlation between the surface state and sensor performance is proposed.     

快速升温法制备CdS纳米带的生长机理研究

在无任何催化剂的条件下, 采用快速升温法在单晶硅衬底上制备了高质量的、形貌均匀的CdS纳米带. X射线衍射(XRD)、透射电镜(TEM)和场发射扫描电镜(FESEM)分析显示, 纳米带属六方单晶结构, 生长方向为[001]. 讨论了纳米带形成的机理, 认为CdS纳米带状六方结构的形成, 主要是由于生长速度的各向异性及在沉积区具有较低的过饱和度和较高的沉积温度等因素导致.