Ultrafast and high-sensitive photodetectors operating from the deep-ultraviolet to near-infrared region at room temperature are essential for many applications such as analytical chemistry, optical positioning, biomedical imaging, and remote sensing. Toward high-performance photodetectors, hybrid colloid quantum dots/graphene photodetectors based on the photogating mechanism have been intensively studied. An ultrahigh sensitivity has been obtained in the range from 105 to 109 A/W but the major challenge of these configurations is the slow operating speed on the millisecond to second time scale. Manipulating the transferring of carriers at the interface of semiconductor nanostructures and graphene is an essential key to optimize the interfacial photogating effect. Here, we grow an absorber layer directly on graphene by e-beam evaporation to obtain a fast photoresponse time of the detectors. Thus, the gap between the high responsivity and fast response time can be bridged. The photodetectors indicate a high photoresponsivity of ∼2.5 × 106 A/W at low incident intensity on the order of femto-watts, a specific detectivity of ∼8.5 × 1011 Jones, and a fast response process of ∼20 ns together with a slow component of ∼850 ns (response time of <1 μs) under a drain-to-source bias of 100 mV. The study has provided a method to obtain high-performance photodetectors based on graphene with high responsivity and fast response time.

Large-scale optoelectronics integration is strongly limited by the lack of efficient light sources, which could be integrated with the silicon complementary metal-oxide-semiconductor (CMOS) technology. Persistent efforts continue to achieve efficient light emission from silicon in the extending the silicon technology into fully integrated optoelectronic circuits. Here, we report the realization of room-temperature stimulated emission in the technologically crucial 1.5 µm wavelength range from Er-doped GaN multiple-quantum wells on silicon and sapphire. Employing the well-acknowledged variable stripe technique, we have demonstrated an optical gain up to 170 cm -1 in the multiple-quantum well structures. The observation of the stimulated emission is accompanied by the characteristic threshold behavior of emission intensity as a function of pump fluence, spectral linewidth narrowing and excitation length. The demonstration of roomtemperature lasing at the minimum loss window of optical fibers and in the eye-safe wavelength region of 1.5 µm are highly sought-after for use in many applications including defense, industrial processing, communication, medicine, spectroscopy and imaging. As the synthesis of Er-doped GaN epitaxial layers on silicon and sapphire has been successfully demonstrated, the results laid the foundation for achieving hybrid GaN-Si lasers providing a new pathway towards full photonic integration for silicon optoelectronics.1 Driven by the strong need for cheap and integrable Si-based optoelectronic devices for a wide range of applications, continuing endeavors have been made to develop structures for light emission, modulation, and detection in this material system. Recent breakthroughs including the demonstration of a high-speed optical modulator in Si, 1-2 photodetectors 3 , and waveguides 4 have brought the concept of transition from electrical to optical interconnects closer to realization.However, the base for silicon photonics, namely, a group IV laser source, still has to be developed.Due to the relatively small and indirect band gap, silicon is a poor light emitter. Nevertheless, lasing devices based on Si have been demonstrated including Si-based impurity lasers, 5 a Raman laser, 6 Si nanocrystals, 7 nanopatterned crystalline Si, 8 GeSn alloy on Si, 9 Ge dots in Si, 10-11 and InGaAs/GaAs nanolasers grown on Si. 12 However, these prototype devices essentially lack the advantages associated with the silicon system by requiring an external pump laser source or function only at low temperatures. While room-temperature luminescence has been realized, 7, 13 population inversion and optical gain have been under discussion and fundamental problems remain.The incorporation of rare earth elements into semiconductor hosts gives rise to sharp, atomiclike and temperature independent emission lines under either optical or electrical excitation. [14][15][16][17][18] Er ions with intra-4f shell transitions from its first excited state ( 4 I 13/2 ) to the ground state ( 4 I 15/2 ) produce 1.5 µm emissio...

We report the 1/f noise characteristics at low-frequency in graphene field-effect transistors that utilized a high- dielectric tantalum oxide encapsulated layer (a few nanometers thick) placed by atomic layer deposition on Si 3 N 4 . A low-noise level of ~ 2.2  10 -10 Hz -1 has been obtained at f = 10 Hz. The origin and physical mechanism of the noise can be interpreted by the McWhorter context, where fluctuations in the carrier number contribute dominantly to the low-frequency noise. Optimizing fabrication processes reduced the number of charged impurities in the graphene field-effect transistors. The study has provided insights into the underlying physical mechanisms of the noise at low-frequency for reducing the noise in graphene-based devices.

We first argue that the covalent bond and the various closed-shell interactions can be thought of as symmetry broken versions of one and the same interaction, viz., the multi-center bond. We use specially chosen molecular units to show that the symmetry breaking is controlled by density and electronegativity variation. We show that the bond order changes with bond deformation but in a step-like fashion, regions of near constancy separated by electronic localization transitions. These will often cause displacive transitions as well so that the bond strength, order, and length are established self-consistently. We further argue on the inherent relation of the covalent, closed-shell, and multi-center interactions with ionic and metallic bonding. All of these interactions can be viewed as distinct sectors on a phase diagram with density and electronegativity variation as control variables; the ionic and covalent/secondary sectors are associated with on-site and bond-order charge density wave respectively, the metallic sector with an electronic fluid. While displaying a contiguity at low densities, the metallic and ionic interactions represent distinct phases separated by discontinuous transitions at sufficiently high densities. Multi-center interactions emerge as a hybrid of the metallic and ionic bond that results from spatial coexistence of delocalized and localized electrons. In the present description, the issue of the stability of a compound is that of mutual miscibility of electronic fluids with distinct degrees of electron localization, supra-atomic ordering in complex inorganic compounds comes about naturally. The notions of electronic localization advanced hereby suggest a high throughput, automated procedure for screening candidate compounds and structures with regard to stability, without the need for computationally costly geometric optimization. I. MOTIVATIONChemical bonding is traditionally discussed in terms of the covalent, ionic, and metallic bond 1 , and weaker, closed-shell interactions such as secondary, donoracceptor, hydrogen, and van der Waals.2,3 The distinction between these canonical bond types is not always clear-cut. For instance, a directional, multi-center 4 bond holding together identical atoms has an inherent ionic feature: In a three-center, linear ppσ bond, 5,6 the central atom contributes only a half orbital to each of the individual ppσ bonds.7 The terminal atoms, on the other hand, each contribute one full orbital, implying a non-uniform charge distribution over the bond. At the same time, the three-center ppσ bond can be thought of as a limiting case of the metallic bond, since the appropriate electron count for an infinite chain corresponds to a half-filled band.4 This identification is consistent with the metallic luster of compounds in which covalent and secondary bonds are comparable in length.3 . In solid-state contexts, interplay between ionic and covalent interactions is often discussed using the van Arkel-Ketelaar triangle for binary compounds;8-11 or revealed, for insta...

The goal of this effort is to establish the conditions and limits under which the Huygens-Fresnel principle accurately describes diffraction in the Monte Carlo ray-trace (MCRT) environment. This goal is achieved by systematic intercomparison of dedicated experimental, theoretical, and numerical results. We evaluate the success of the Huygens-Fresnel principle by predicting and carefully measuring the diffraction fringes produced by both single slit and circular apertures. We then compare the results from the analytical and numerical approaches with each other and with dedicated experimental results. We conclude that use of the MCRT method to accurately describe diffraction requires that careful attention be paid to the interplay among the number of aperture points, the number of rays traced per aperture point, and the number of bins on the screen. This conclusion is supported by standard statistical analysis, including the adjusted coefficient of determination, Radj2, the rms deviation, and the reduced chi-square statistic, χv2.