Strong confinement of optical fields using localized surface phonon polaritons in cubic boron nitride.

Phonon polaritons (PhPs) are long-lived electromagnetic modes that originate from the coupling of infrared (IR) photons with the bound ionic lattice of a polar crystal. Cubic-boron nitride (cBN) is such a polar, semiconductor material which, due to the light atomic masses, can support high-frequency optical phonons. Here we report on random arrays of cBN nanostructures fabricated via an unpatterned reactive ion etching process. Fourier-transform infrared reflection spectra suggest the presence of localized surface PhPs within the reststrahlen band, with quality factors in excess of 38 observed. These can provide the basis of next-generation IR optical components such as antennas for communication, improved chemical spectroscopies, and enhanced emitters, sources, and detectors.

Chemical Vapor Deposition Growth of Large Single-Crystal Mono-, Bi-, Tri-Layer Hexagonal Boron Nitride and Their Interlayer Stacking.

Two-dimensional hexagonal boron nitride (h-BN) is a wide bandgap material which has promising mechanical and optical properties. Here we report the realization of an initial nucleation density of h-BN <1 per mm2 using low-pressure chemical vapor deposition (CVD) on polycrystalline copper. This enabled wafer-scale CVD growth of single-crystal monolayer h-BN with a lateral size up to ∼300 µm, bilayer h-BN with a lateral size up to ∼60 µm, and trilayer h-BN with a lateral size up to ∼35 µm. Based on the large single-crystal monolayer h-BN domain, the sizes of the as-grown bi- and trilayer h-BN grains are 2 orders of magnitude larger than typical h-BN multilayer domains. In addition, we achieved coalesced h-BN films with an average grain size ∼100 µm. Various flake morphologies and their interlayer stacking configurations of bi- and trilayer h-BN domains were studied. Raman signatures of mono- and multilayer h-BN were investigated side by side in the same film. It was found that the Raman peak intensity can be used as a marker for the number of layers.

Temperature Dependence of Wavelength Selectable Zero-Phonon Emission from Single Defects in Hexagonal Boron Nitride.

We investigate the distribution and temperature-dependent optical properties of sharp, zero-phonon emission from defect-based single photon sources in multilayer hexagonal boron nitride (h-BN) flakes. We observe sharp emission lines from optically active defects distributed across an energy range that exceeds 500 meV. Spectrally resolved photon-correlation measurements verify single photon emission, even when multiple emission lines are simultaneously excited within the same h-BN flake. We also present a detailed study of the temperature-dependent line width, spectral energy shift, and intensity for two different zero-phonon lines centered at 575 and 682 nm, which reveals a nearly identical temperature dependence despite a large difference in transition energy. Our temperature-dependent results are well described by a lattice vibration model that considers piezoelectric coupling to in-plane phonons. Finally, polarization spectroscopy measurements suggest that whereas the 575 nm emission line is directly excited by 532 nm excitation, the 682 nm line is excited indirectly.

Surface Functionalized Graphene Biosensor on Sapphire for Cancer Cell Detection.

Graphene has several unique physical, optical and electrical properties such as a two-dimensional (2D) planar structure, high optical transparency and high carrier mobility at room temperature. These make graphene interesting for electrical biosensing. Using a catalyst-free chemical vapor deposition (CVD) method, graphene film is grown on a sapphire substrate. There is a single or a few sheets as confirmed by Raman spectroscopy and atomic force microscopy (AFM). Electrical graphene biosensors are fabricated to detect large-sized biological analytes such as cancer cells. Human colorectal carcinoma cells are sensed by the resistance change of an active bio-functionalized graphene device as the cells are captured by the immobilized antibody surface. The functionalized sensors show an increase in resistance as large as ~20% of the baseline with a small number of adhered cells. This study suggests that the bio-functionalized electrical graphene sensors on sapphire, which is a highly transparent material, can potentially detect circulating tumor cells (CTCs) and monitor cellular electrical behavior while being compatible with fluorescence-based optical-detection bioassays.

Electrical characteristics of multilayer MoS2 FET's with MoS2/graphene heterojunction contacts.

The electrical properties of multilayer MoS2/graphene heterojunction transistors are investigated. Temperature-dependent I-V measurements indicate the concentration of unintentional donors in exfoliated MoS2 to be 3.57 × 10(11) cm(-2), while the ionized donor concentration is determined as 3.61 × 10(10) cm(-2). The temperature-dependent measurements also reveal two dominant donor levels, one at 0.27 eV below the conduction band and another located at 0.05 eV below the conduction band. The I-V characteristics are asymmetric with drain bias voltage and dependent on the junction used for the source or drain contact. I-V characteristics of the device are consistent with a long channel one-dimensional field-effect transistor model with Schottky contact. Utilizing devices, which have both graphene/MoS2 and Ti/MoS2 contacts, the Schottky barrier heights of both interfaces are measured. The charge transport mechanism in both junctions was determined to be either thermionic-field emission or field emission depending on bias voltage and temperature. On the basis of a thermionic field emission model, the barrier height at the graphene/MoS2 interface was determined to be 0.23 eV, while the barrier height at the Ti/MoS2 interface was 0.40 eV. The value of Ti/MoS2 barrier is higher than previously reported values, which did not include the effects of thermionic field emission.

Metal free growth of graphene on quartz substrate using chemical vapor deposition (CVD).

Graphene was grown on (0001) quartz substrate (z-cut) using catalyst free Chemical Vapor Deposition (CVD). Methane was used as a carbon source and hydrogen was introduced independently to optimize the growth. The effect of growth temperature was investigated while varying the temperature between 1000 and 1300 degrees C. With an optimized condition, a thin (< or = 2 mono-layer) continuous graphene film was grown as confirmed by Raman spectroscopy, optical transmission, and electrical measurements. The best quality film showed the Raman D-peak to G-peak intensity ratio of approximately 0.8 with the 2D-peak width of approximately 60 cm(-1). High resolution X-ray Photoelectron Spectroscopy (XPS) revealed that the grown graphene is slightly oxidized but there is no detectable Si--C chemical bond in the graphene/quartz system. Hall effect measurements exhibited a carrier mobility of approximately 400 cm2/V x s with a sheet carrier density of approximately 5 x 10(12) cm(-2).

A universal transfer route for graphene.

Often synthetic graphene requires transfer onto an arbitrary substrate prior to use because the substrate it was originally synthesized on is inappropriate for either electrical measurement or characterization. While a variety of routes have been developed they are substrate dependant and often involve the use of harsh treatments. Here we present a facile and cheap route that can be applied to graphene over any substrate. This universal transfer route is based on a wet chemical reaction producing gaseous species which can intercalate between the substrate and the graphene and thus gently delaminate the two.

Progress, challenges, and opportunities in two-dimensional materials beyond graphene.

Graphene's success has shown that it is possible to create stable, single and few-atom-thick layers of van der Waals materials, and also that these materials can exhibit fascinating and technologically useful properties. Here we review the state-of-the-art of 2D materials beyond graphene. Initially, we will outline the different chemical classes of 2D materials and discuss the various strategies to prepare single-layer, few-layer, and multilayer assembly materials in solution, on substrates, and on the wafer scale. Additionally, we present an experimental guide for identifying and characterizing single-layer-thick materials, as well as outlining emerging techniques that yield both local and global information. We describe the differences that occur in the electronic structure between the bulk and the single layer and discuss various methods of tuning their electronic properties by manipulating the surface. Finally, we highlight the properties and advantages of single-, few-, and many-layer 2D materials in field-effect transistors, spin- and valley-tronics, thermoelectrics, and topological insulators, among many other applications.

van der Waals epitaxial growth of graphene on sapphire by chemical vapor deposition without a metal catalyst.

van der Waals epitaxial growth of graphene on c-plane (0001) sapphire by CVD without a metal catalyst is presented. The effects of CH(4) partial pressure, growth temperature, and H(2)/CH(4) ratio were investigated and growth conditions optimized. The formation of monolayer graphene was shown by Raman spectroscopy, optical transmission, grazing incidence X-ray diffraction (GIXRD), and low voltage transmission electron microscopy (LVTEM). Electrical analysis revealed that a room temperature Hall mobility above 2000 cm(2)/V·s was achieved, and the mobility and carrier type were correlated to growth conditions. Both GIXRD and LVTEM studies confirm a dominant crystal orientation (principally graphene [10-10] || sapphire [11-20]) for about 80-90% of the material concomitant with epitaxial growth. The initial phase of the nucleation and the lateral growth from the nucleation seeds were observed using atomic force microscopy. The initial nuclei density was ~24 µm(-2), and a lateral growth rate of ~82 nm/min was determined. Density functional theory calculations reveal that the binding between graphene and sapphire is dominated by weak dispersion interactions and indicate that the epitaxial relation as observed by GIXRD is due to preferential binding of small molecules on sapphire during early stages of graphene formation.

Improvement of carrier mobility of top-gated SiC epitaxial graphene transistors using a PVA dielectric buffer layer.

The effects of treatment with polyvinyl alcohol (PVA) and a dielectric film of HfO(2) on the properties of SiC based epitaxial graphene have been explored and analyzed. We have characterized the carrier mobility of graphene on Si-face and C-face SiC with a layer of HfO(2), with or without an initial PVA treatment on the device active layer. Epitaxial graphene grown on the C-face displays a higher mobility than a film grown on the silicon face. Also, the mobility in the presence of the PVA treatment with HfO(2) dielectric layer has been improved, compared with the mobility after deposition of only gate dielectric: ∼20% in C-face graphene and ∼90% in Si-face graphene. This is a major improvement over the degradation normally observed with dielectric/graphene systems.

SiC surface orientation and Si loss rate effects on epitaxial graphene.

We have explored the properties of SiC-based epitaxial graphene grown in a cold wall UHV chamber. The effects of the SiC surface orientation and silicon loss rate were investigated by comparing the characteristics of each formed graphene. Graphene was grown by thermal decomposition on both the silicon (0001) and carbon (000-1) faces of on-axis semi-insulating 6H-SiC with a "face-down" and "face-up" orientations. The thermal gradient, in relation to the silicon flux from the surface, was towards the surface and away from the surface, respectively, in the two configurations. Raman results indicate the disorder characteristics represented by ID/IG down to < 0.02 in Si-face samples and < 0.05 in C-faces over the 1 cm2 wafer surface grown at 1,450°C. AFM examination shows a better morphology in face-down surfaces. This study suggests that the optimum configuration slows the thermal decomposition and allows the graphene to form near the equilibrium. The Si-face-down orientation (in opposition to the temperature gradient) results in a better combination of low disorder ratio, ID/IG, and smooth surface morphology. Mobility of Si-face-down orientation has been measured as high as approximately 1,500 cm2/Vs at room temperature. Additionally, the field effect transistors have been fabricated on both Si-face-down and C-face-down showing an ambipolar behavior with more favorable electron conduction.

Oriented 2D covalent organic framework thin films on single-layer graphene.

Covalent organic frameworks (COFs), in which molecular building blocks form robust microporous networks, are usually synthesized as insoluble and unprocessable powders. We have grown two-dimensional (2D) COF films on single-layer graphene (SLG) under operationally simple solvothermal conditions. The layered films stack normal to the SLG surface and show improved crystallinity compared with COF powders. We used SLG surfaces supported on copper, silicon carbide, and transparent fused silica (SiO(2)) substrates, enabling optical spectroscopy of COFs in transmission mode. Three chemically distinct COF films grown on SLG exhibit similar vertical alignment and long-range order, and two of these are of interest for organic electronic devices for which thin-film formation is a prerequisite for characterizing their optoelectronic properties.

Epitaxial growth of graphitic carbon on C-face SiC and Sapphire by chemical vapor deposition (CVD).

Epitaxial, graphitic carbon thin films were directly grown on C-face/ (0001̄) SiC and (0001) sapphire by chemical vapor deposition (CVD), using propane as a carbon source and without any catalytic metal on the substrate surface. Raman spectroscopy shows the signature of multilayer graphene/graphite growth on both the SiC and sapphire. Raman 2D-peaks have Lorentzian lineshapes with FWHM of ~60 cm(-1) and the ratio of the D-peak to G-peak intensity (I(D)/I(G)) linearly decreases (down to 0.06) as growth temperature is increased. The epitaxial relationship between film and substrates were determined by x-ray diffraction. On both substrates, graphitic layers are oriented parallel to the substrate, but exhibit significant rotational disorder about the surface normal, and predominantly rhombohedral stacking. Film thicknesses were determined to be a function of growth time, growth temperature, and propane flow rate.

Electrochemical impedance biosensor with electrode pixels for precise counting of CD4+ cells: a microchip for quantitative diagnosis of HIV infection status of AIDS patients.

Oriented for the quantitative diagnosis of HIV infection status of AIDS patients, a cell biosensor based on electrochemical impedance spectroscopy has been developed for the precise counting of human CD4(+) cells. In this new biosensor, the sensing area was composed of densely packed working electrode pixels, each of which was comparable to a single CD4(+) cell in size. CD4(+) cells were captured on the chemically modified electrode pixels, and detected individually by monitoring the interfacial impedance changes on each independent pixel. The detection of a single cell was achieved by the "on" and "off" states of electrode pixel, depending on the cell capture status. The cell counting was digitalized by summing the electrode pixels in the "on" state (captured with a single cell). Compared with peer counting methods, the biosensor reported here was featured with a small device dimension, a minimal sample consumption, a finest detection resolution and a highest counting accuracy.

Free-standing epitaxial graphene.

We report on a method to produce free-standing graphene sheets from epitaxial graphene on silicon carbide (SiC) substrate. Doubly clamped nanomechanical resonators with lengths up to 20 microm were patterned using this technique and their resonant motion was actuated and detected optically. Resonance frequencies of the order of tens of megahertz were measured for most devices, indicating that the resonators are much stiffer than expected for beams under no tension. Raman spectroscopy suggests that the graphene is not chemically modified during the release of the devices, demonstrating that the technique is a robust means of fabricating large-area suspended graphene structures.

Ultrafast optical-pump terahertz-probe spectroscopy of the carrier relaxation and recombination dynamics in epitaxial graphene.

The ultrafast relaxation and recombination dynamics of photogenerated electrons and holes in epitaxial graphene are studied using optical-pump terahertz-probe spectroscopy. The conductivity in graphene at terahertz frequencies depends on the carrier concentration as well as the carrier distribution in energy. Time-resolved studies of the conductivity can therefore be used to probe the dynamics associated with carrier intraband relaxation and interband recombination. We report the electron-hole recombination times in epitaxial graphene for the first time. Our results show that carrier cooling occurs on subpicosecond time scales and that interband recombination times are carrier density dependent.

Distributions of conduction electrons as manifested in MAS NMR of gallium nitride.

A strategy is demonstrated for identifying unambiguously and characterizing quantitatively the effects of distributions of conduction electron concentrations arising from intentional or unintentional dopants in semiconductors by magic-angle spinning (MAS) NMR. The 71Ga MAS NMR spectra of a number of chemically synthesized GaN samples with no intentional doping show inhomogeneously broadened absorptions to high frequency of the main peak. These broad signals are shown, from spin-lattice relaxation time measurements as a function of shift position in a single sample, to be due to Knight shifts arising from degenerate conduction electrons. For a GaN sample with Ge as an intentional dopant at the 0.13% (wt) level, the spectrum is dramatically broadened and shifted to high frequency by up to several hundred parts per million. Analysis of the inhomogeneously broadened line shape yields a quantitative probability density function for electron carrier concentration in the bulk sample that reflects significant compositional heterogeneity due to a variety of possible sources.

Development and characterization of on-chip biopolymer membranes.

In lab-on-a-chip applications, filtration is currently performed prior to sample loading or through pre-cast membranes adhered to the substrate. These membranes cannot be patterned to micrometer resolution, and their adhesion may be incompatible with the fabrication process or may introduce contaminants. We have developed an on-chip separation process using a biocompatible polymer that can be patterned and has controllable molecular rejection properties. We spun cast cellulose acetate (CA) membranes directly onto silicon wafers. Characterization of the molecular flux across the membrane showed that molecular weight and charge are major factors contributing to the membranes' rejection characteristics. Altering casting conditions such as polymer concentration in the casting solution and the quenching-bath composition and/or temperature allowed control of the molecular weight cut-off (MWCO). Three MWCOs; 300, 350, and 700 Da have been achieved for non-linear molecules. Molecular shape is also very important as much higher molecular weight single-stranded DNA was electrophoresed across the membranes while heme with a similar negative charge density was rejected. This was due to DNA's small molecular cross section. This is an important result because heme inhibits polymerase chain reactions (PCR) reducing the detection and characterization of DNA from blood samples.

Carcinoid tumours of the larynx.

The larynx is a rare site of origin of neuroendocrine carcinomas. They can be divided into typical, atypical and small cell tumours on the basis of their histopathological differentiation. The tumour histology and prognosis correlate closely. The typical carcinoid tumours are well differentiated with a benign course. Conservative surgery for local disease is the treatment and is associated with good survival. The atypical carcinoid tumours are poorly differentiated with an aggressive course. Response to radiotherapy and chemotherapy is poor. The treatment of choice is adequate total excision of the lesion with neck dissection if there is clinical evidence of cervical lymphadenopathy and a careful follow-up so as to recognise and treat any metastatic spread. We present two cases with similar history and clinical findings-one typical and the other atypical-and discuss the relevant literature.