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The bonding and electronic properties of In

Epigraphene, which is graphene that is epitaxially grown on a single crystal silicon carbide (SiC) substrate, was proposed as a path to extend Moore’s roadmap beyond silicon at the beginning of the millennium. Despite considerable progress, the lack of a bandgap in graphene and steps on the substrate continued to be roadblocks. Here we show a new method to produce millimeter scale step-free terraces covered with a graphene-like interface that is bonded to the SiC surface. This so-called buffer layer is found to be two-dimensional semiconducting epigraphene (SEG) with a 0.6 eV bandgap and a room temperature mobility exceeding 4000 cm2V-1s-1, which surpasses all current 2D single layer semiconductors by a factor of 10. A top-gated SEG field-effect transistor demonstrates an on-to-off ratio of 104 which is suitable for digital electronics. In addition, we also find that hydrogen intercalation converts SEG into a high-mobility semi-metallic epigraphene monolayer that can be seamlessly integrated with SEG. Centimeter scale mean free paths are observed in the epigraphene edge state of this quasi-freestanding monolayer, which is by a factor of 1000, the largest room temperature electronic mean free path observed in any material.

We present a systematic study of the structures and electronic properties of vanadium-doped silicon cluster anions, VSin− (n = 14–20), by combining photoelectron spectroscopy (PES) measurements and density functional theory (DFT) calculations.

We investigated the structural evolution and electronic properties of medium-sized silicon cluster anions doped with two transition metal atoms, TM2Sin− (TM = V, Cr; n = 14–20), by using mass-selective anion photoelectron spectroscopy combined with density functional theory (DFT) calculations.

A cryogenic beam apparatus for studying neutral clusters has been built and tested. The lowest beam temperature reaches less than 9 K at a repetition rate of 20 Hz. Mechanical decoupling from the refrigerator avoids misalignment during temperature ramping. Adopting a permanent magnet based magnetic deflector eliminates the hysteresis and electric noise of the traditional electromagnet and offers excellent reproducibility of the applied magnetic field. The mass spectrometer can operate in either Mass Spectroscopy Time-Of-Flight mode or Position-Sensitive Time-Of-Flight mode with spatial resolution better than 7 μm. Its performance is demonstrated with niobium and cobalt clusters.

Subwavelength chiral metamaterials with tunable geometries and compositions are essential to advance the development of chiral biochemical samples detection. Here, we report a spatial symmetry breaking chiral terahertz (THz) metamaterial structure with stacked layers of L-shape arranged gold disks as the periodic unit cell. The chiroptical response can be adjusted on-demand by manipulating the number of stacking layers and the twisted angle of the periodic unit between adjacent array layers. We reveal that the chiroptical response originates from the optical resonances of the gold disks and the adjacent gold disks array layers via experiments and numerical simulation analysis. Furthermore, we find that this chiral metamaterial can realize label-free detection of proline in biological samples and label-free enantio-discrimination of chiral molecules. The change of the analyte concentration can also regulate the transmission circular dichroism (TCD) intensity of the chiral metamaterials. Our results not only provide new ideas into the design of functional chiral metamaterials, but also bring new strategies to develop chiroptical biosensing devices.

Open in a separate window Nitrogen dioxide (NO2) is one of the most dangerous air pollutants that can affect human health even at the ppb (part per billion) level. Thus, the superior sensing performance of nitrogen dioxide gas sensors is an imperative for real-time environmental monitoring. Traditional solid-state sensors based on metal-oxide transistors have the drawbacks of high power consumption, high operating temperature, poor selectivity, and difficult integration with other electronics. In that respect, graphene-based gas sensors have been extensively studied as potential replacements. However, their advantages of high sensing efficiency, low power consumption, and simple electronic integration have been countered by their slow response and poor repeatability. Here, we report the fabrication of high-performance ultraviolet (UV)-assisted room temperature NO2 sensors based on chemical vapor deposition-grown graphene. UV irradiation improves the response of the sensor sevenfold with respect to the dark condition attaining 26% change in resistance at 100 ppm NO2 concentration with a practical detection limit below 1 ppm (42.18 ppb). In addition, the recovery time was shortened fivefold to a few minutes and the excellent repeatability. This work may provide a promising and practical method to mass produce room-temperature NO2 gas sensors for real-time environment monitoring due to its simple fabrication process, low cost, and practicality.

Static dipole polarizabilities for the first 20 atoms and ions of the Periodic Table are calculated within a single theoretical scheme: the coupled-cluster method with a Hartree–Fock reference wavefunction using an augmented-polarization-consistent and polarization-consistent basis set. The values of the atomic polarizabilities of neutral atoms obtained here agree extremely well with the experimentally measured values. For the ions, all the calculations are consistent with experimental values with the highest resolution so far, except for the monovalent calcium ion, probably due to strong relativistic effects. To the best of our knowledge, this is the first time, applying a single theoretical scheme successfully predicts static polarizabilities of all 20 consecutive elements across 8 columns and more than 3 full periods being consistent with the experimentally measured values both for neutrals and ions. The results clearly signal the universality of this theoretical scheme for the atomic polarizability calculation, and should be easy to extend to other elements with weak relativistic effects. We adopt the CCSD(T) method with an augmented polarization-consistent and polarization-consistent basis set to calculate the atomic and ionic polarizabilities for the first 20 atoms and ions of the Periodic Table. Our calculation results are in good agreement with the experimental data with the highest accuracy so far and the reported high accuracy calculation results. Semi-logarithm plot of calculated static polarizabilities of neutral atoms (filled circles, black line) and ions (open circles). The colored lines join isoelectronic species.

High sensitivity biosensors based on the coupling of surface plasmon polaritons on titanium nitride (TiN) and a planar waveguide mode were built, they were proved by sensing three different media: air, water and dried egg white, sensors described here could be useful for sensing materials with a refractive index between 1.0 and 1.6, in particular, materials of biological interest with a refractive index in the range 1.3&ndash, 1.6, like those containing biotin and/or streptavidin. They were built by depositing Nb2O5/SiO2/TiN multilayer structures on the flat surface of D-shaped sapphire prisms by using the dc magnetron sputtering technique. Attenuated total reflection (ATR) experiments in the Kretschmann configuration were accomplished for the air/TiN/Prism and S/Nb2O5/SiO2/TiN/Prism structures, S being the sample or sensing medium. ATR spectra for plasmons at the TiN/air interface showed a broad absorption band for angles of incidence between 36 and 85&deg, with full width at half maximum (FWHM) of approximately 40&deg, For the S/Nb2O5/SiO2/TiN/Prism structures, ATR spectra showed a sharp reflectivity peak, within the broad plasmonic absorption band, which was associated with Fano resonances. The angular position and FWHM of the Fano resonances strongly depend on the refractive index of the sensing medium. ATR spectra were fitted by using the transfer-matrix method. Additionally, we found that angular sensitivity and figure of merit increase with increasing the refractive index of the sensing medium.

Biosensors for quick diagnosis and in situ monitoring are increasingly needed in health care. Field-effect transistor (FET) based biosensors have attracted much attention due to their high sensitivity and compatibility with point-of-care applications. As the most important 2D material, graphene has been investigated intensively as a channel material for transistor-based sensors due to its easily enhanced selectivity by rather simple functionalization. However, in order to realize its practical applications, challenges still remain, such as device stability and reproducibility. Here, we review recent progress in the general design strategy of high-performance graphene field-effect transistor (GFET) biosensors with emphasis on the device physics, defects, Debye screening, and functionalization. Finally, both current applications and perspectives on future development are given.

Refractive index sensors of high sensitivity were built by depositing thin films of titanium nitride and plane waveguides made of Nb 2 O 5 and SiO 2 on the planar face of D-shaped prisms, by dc and rf magnetron sputtering. Devices with structures Glass/TiN, Sapphire/TiN and Sapphire/TiN/SiO 2 /Nb 2 O 5 were studied. Measurements of reflectance as a function of the angle of incidence were made in the Kretschmann configuration. For devices Glass/TiN and Sapphire/TiN, typical broad absorption band was observed. For devices with structure Sapphire/TiN/SiO 2 /Nb 2 O 5 , we observed a sharp reflectance peak, at a very well defined value of the angle of incidence within the broad absorption band, attributed to the phenomena of Plasmon Induced Transparency (PIT) and Fano resonance. Then, in the range of incidence angle values for which PIT and Fano resonance were observed, we studied the spectral response in the range of wavelength between 400 and 800 nm. Results show that high sensitivity sensors may also be used as tunable-by-incidence-angle optical filters. All data, reflectance vs angle of incidence and reflectance vs wavelength, were fitted by using a calculation program based on the solutions to Maxwell equations and boundary conditions.

Pulse delay generators are ubiquitous in laboratories to coordinate and control the timing between different devices in applications that include lasers, mass spectrometers, and other scientific instruments. The most important required characteristics are precision, to control time exactly, and low-jitter, to minimize uncertainty in experiments. Here, we introduce a new design of a high precision and low-jitter digital delay generator based on a Field Programmable Gate Array (FPGA). The final delay is composed of steps of 4.2 ns (coarse delay) with fine steps of 16 ps (fine delay). The coarse delay is generated by a 240 MHz pulse sequence from the FPGA with a 50 MHz clock. An embedded time-to-digital conversion unit is used to measure the interval between the external trigger and the clock signal, which, together with an integrated delay generator, is used to realize the fine delay. Jitter compensation is achieved through a measurement-and-feedback module. A computer interface is designed to control the system through a Nios II processor. Measurements confirm a time resolution of 16 ± 2 ps with jitter below 450 ± 20 ps (at 24 °C) with a maximum delay of 1 s. The whole system is simple in structure and low in cost.

Cryogenic molecular beam experiments show that the ferroelectric dipole moments of small niobium clusters with an even number of valence electrons n are typically greater than those with odd n. This is verified in alloy clusters NbNXM where X=Al, Au, O, Mn, Fe, and Co; N≤100;M≤3. Like in superconducting alloys, Mn doping quenches the effect while Al and Au enhance it, suggesting a relation between cluster ferroelectricity and bulk superconductivity. A correlated ground state is proposed where the even-odd effect is caused by the depolarizing effect of a single unpaired electron.

Individual amino acid molecules embedded in helium nanodroplets fragment extensively when the beam is ionized by electron bombardment. However, we find that when glycine and tryptophan are picked up right after, or right before, a small amount of water, the mass spectra become significantly altered. For glycine, the detected ions consist almost entirely of intact protonated amino acids, with or without a few water molecules attached. In other words, the presence of water exerts a striking “buffering” effect on the ionization-induced fragmentation. For tryptophan the effect is weaker but also present. In both cases, the hydroxyl group lost upon ionization overwhelmingly comes from the water partner (in strong contrast to the situation observed when amino acids are picked up by neat water clusters). A complementary experiment involving DCl molecules co-embedded with water shows that in this case Cl and/or DCl invariably leave the droplet upon ionization. The observed patterns may be steered by the analytes' dipole moments or by solvation effects.

Advanced pulsed cryogenic molecular beam electric deflection methods involving position-sensitive mass spectrometry and 7.87 eV ionizing radiation were used for the first time to measure the polarizabilities of more than half of the metallic elements in the periodic table.

Electric deflections of gas-phase, cryogenically cooled, neutral niobium clusters [Nb N ; number of atoms ( N ) = 2 to 150, temperature ( T ) = 20to 300kelvin], measured in molecular beams, show that cold clusters may attain an anomalous component with very large electric dipole moments. In contrast, room-temperature measurements show normal metallic polarizabilities. Characteristic energies k B T G ( N ) [Boltzmann constant k B times a transition temperature T G ( N )] are identified, below which the ferroelectric-like state develops. Generally, T G decreases [110 > T G ( N ) > 10K] as N increases, with pronounced even-odd alternations for N > 38. This new state of metallic matter may be related to bulk superconductivity.

Simultaneous magnetic and electric deflection measurements of rhodium clusters (Rh(N), 6 ≤ N ≤ 40) reveal ferromagnetism and ferroelectricity at low temperatures, while neither property exists in the bulk metal. Temperature-independent magnetic moments (up to 1 μ(B) per atom) are observed, and superparamagnetic blocking temperatures up to 20 K. Ferroelectric dipole moments on the order of 1D with transition temperatures up to 30 K are observed. Ferromagnetism and ferroelectricity coexist in rhodium clusters in the measured size range, with size-dependent variations in the transition temperatures that tend to be anticorrelated in the range n = 6-25. Both effects diminish with size and essentially vanish at N = 40. The ferroelectric properties suggest a Jahn-Teller ground state. These experiments represent the first example of multiferroic behavior in pure metal clusters.

Sodium nanoparticles of ∼10 nm radius, generated in a vapor condensation source, produce copious amounts of positive ions upon colliding with a surface. The ion flux does not display an exponential surface temperature dependence, is present for both conducting and non-conducting surfaces, and carries a substantial kinetic energy. On the basis of these characteristics it is concluded that the signal arises from dissociative ionization produced by energetic impact, rather than from thermal charge transfer to the surface.

Water clusters embedding a nitric acid molecule HNO3(H2O)_{n=1-10} are investigated via electrostatic deflection of a molecular beam. We observe large paraelectric susceptibilities that greatly exceed the electronic polarizability, revealing the contribution of permanent dipole moments. The moments derived from the data are also significantly higher than those of pure water clusters. An enhancement in the susceptibility for n=5,6 and a rise in cluster abundances setting in at n=6 suggest that dissociation of the solvated acid molecule into ions takes place in this size range., Proceedings of ISACC 2009, The Fourth International Symposium "Atomic Cluster Collisions: structure and dynamics from the nuclear to the biological scale" (AIP Conference Proceedings)

Above a critical temperature, a supported single domain ferromagnetic particle responds to an applied magnetic field as if it were a paramagnet with a very large spin. Its average magnetization is given by the Langevin equation as expected from simple thermodynamic considerations. The average magnetization of an ensemble of unsupported ferromagnetic clusters also approximately follows the Langevin equation even for small clusters in a low-temperature ensemble. The reason is not obvious because there is no heat bath for low-energy clusters so that elementary thermodynamic requirements for the Langevin equation are not satisfied. We investigated the magnetic deflections of cobalt clusters (${\text{Co}}_{N}$, $12\ensuremath{\le}N\ensuremath{\le}200$) using molecular-beam methods over a wide range of temperatures $(20\ensuremath{\le}T\ensuremath{\le}100\text{ }\text{K})$ and magnetic fields $(0\ensuremath{\le}B\ensuremath{\le}2\text{ }\text{T})$. A distribution of magnetization is observed for the cluster beams. Previously, we showed that the average magnetization of the cluster beam follows Langevin function closely for all temperatures and magnetic fields investigated, and proposed an avoided-crossing model that takes into account interacting spin-rotational states. In this paper, we report a comprehensive study of the magnetization distribution and present in depth the avoided-crossing model. The model explains both the average and the width of the magnetization distributions of the cluster beam in terms of the ensemble temperature without requiring that individual clusters have defined temperatures. We also show that the spin-relaxation model is the high-temperature limit of the avoided-crossing model.

The induced polarization of a beam of polar clusters or molecules passing through an electric or magnetic field region differs from the textbook Langevin-Debye susceptibility. This distinction, which is important for the interpretation of deflection and focusing experiments, arises because instead of acquiring thermal equilibrium in the field region, the beam ensemble typically enters the field adiabatically, i.e., with a previously fixed distribution of rotational states. We discuss the orientation of rigid symmetric top systems with a body-fixed electric or magnetic dipole moment. The analytical expression for their "adiabatic-entry" orientation is elucidated and compared with exact numerical results for a range of parameters. The differences between the polarization of thermodynamic and "adiabatic-entry" ensembles of prolate and oblate tops, and of symmetric top and linear rotators, are illustrated and identified. © 2008 American Institute of Physics.

Magnetic moments of ${\mathrm{Co}}_{N}{\mathrm{Mn}}_{M}$ and ${\mathrm{Co}}_{N}{\mathrm{V}}_{M}$ clusters ($N\ensuremath{\le}60$; $M\ensuremath{\le}N/3$) are measured in molecular beams using the Stern-Gerlach deflection method. Surprisingly, the per atom average moments of ${\mathrm{Co}}_{N}{\mathrm{Mn}}_{M}$ clusters are found to increase with Mn concentration, in contrast to bulk CoMn. The enhancement with Mn doping is found to be independent of cluster size and composition in the size range studied. Meanwhile, ${\mathrm{Co}}_{N}{\mathrm{V}}_{M}$ clusters show reduction of average moments with increasing V doping, consistent with what is expected in bulk CoV. The results are discussed within the virtual bound states model.

An inhomogeneous electric field is used to study the deflection of a supersonic beam of water molecules. The deflection profiles show strong broadening accompanied by a small net displacement towards higher electric fields. The profiles are in excellent agreement with a calculation of rotational Stark shifts. The molecular rotational temperature being the only adjustable parameter, beam deflection is found to offer an accurate and practical means of determining this quantity. A pair of especially strongly responding rotational sublevels, adding up to ≈25% of the total beam intensity, are readily separated by deflection, making them potentially useful for further electrostatic manipulation. © 2007 The American Physical Society.

Electric deflections of niobium clusters in molecular beams show that they have permanent electric dipole moments at cryogenic temperatures but not higher temperatures, indicating that they are ferroelectric. Detailed analysis shows that the deflections cannot be explained in terms of a rotating classical dipole, as claimed by Anderson et al. The shapes of the deflected beam profiles and their field and temperature dependences indicates that the clusters can exist in two states, one with a dipole and the other without. Cluster with dipoles occupy lower energy states. Excitations from the lower states to the higher states can be induced by low fluence laser excitation. This causes the dipole to vanish., 25 pages, 7 figures

Water clusters (H(2)O)(n), n

Magnetic properties of free ${\mathrm{Bi}}_{N}{\mathrm{Mn}}_{M}$ clusters ($N=2--20$, $M=0--7$) are determined from Stern-Gerlach deflections at low temperature $(46.5\phantom{\rule{0.3em}{0ex}}\mathrm{K})$. Pure bismuth clusters with odd number of atoms exhibit paramagnetic deflections. The addition of manganese atoms produces a ferromagnetic response which is strongly size dependent. Certain combinations have very large magnetic moments such as ${\mathrm{Bi}}_{5}{\mathrm{Mn}}_{3}$, ${\mathrm{Bi}}_{9}{\mathrm{Mn}}_{4}$, ${\mathrm{Bi}}_{10}{\mathrm{Mn}}_{5}$, and ${\mathrm{Bi}}_{12}{\mathrm{Mn}}_{6}$.

Water clusters (H2On and (D2On (nor=15) are produced by supersonic expansion and then pick up an additional heavy or light water molecule, respectively, or an amino-acid molecule (glycine or tryptophan). The products are analyzed by electron bombardment ionization mass spectrometry. Ionization proceeds via the well-known loss of an OH or OD group, but these turn out to have a strong predilection to come from the guest, rather than the host, molecule: between 30% and 60% of the time the loss originates in the picked-up molecule, even for large n. In fact, the magnitude of this fraction depends on the guest, but is largely insensitive to the cluster size. The observations suggest that the host clusters are frozen into compact annealed shapes, and the adducts reside on the surface and form an inhomogeneity where dissociative ionization tends to localize. It is also notable that no significant amino-acid fragmentation is observed beyond the hydroxyl group loss, in contrast to other measurements employing electron-impact ionization.

Magnetizations and magnetic moments of free cobalt clusters ${\mathrm{Co}}_{N}$ ($12lNl200$) in a cryogenic ($25\text{ }\text{ }\mathrm{K}\ensuremath{\le}T\ensuremath{\le}100\text{ }\text{ }\mathrm{K}$) molecular beam were determined from Stern-Gerlach deflections. All clusters preferentially deflect in the direction of the increasing field and the average magnetization resembles the Langevin function for all cluster sizes even at low temperatures. We demonstrate in the avoided crossing model that the average magnetization may result from adiabatic processes of rotating and vibrating clusters in the magnetic field and that spin relaxation is not involved. This resolves a long-standing problem in the interpretation of cluster beam deflection experiments with implications for nanomagnetic systems in general.

Molecular beam Stern-Gerlach deflection measurements on Nb clusters (Nb(N), N

In experiments in which a cluster beam picks up guest molecules by passing through a cell, the detection efficiency may be limited because of cluster beam scattering by the molecular vapor. We present a modified pick-up cell design that reduces cluster deflections and thereby improves the detection of mixed clusters. Its performance is illustrated with the help of a beam of water clusters picking up amino-acid or heavy-water molecules.